There appears to be no end to the money scandals involving politicians and their aides. On Friday, Mr. Yutaka Inoue, president of the Upper House, resigned amid allegations that his aide took a huge bribe from a construction company. In this year alone, two other legislators surrendered their Diet seats over financial irregularities involving their secretaries.

Just the day before, Mr. Inoue had denied the allegations, brushing aside rumors that he might be forced to quit. The Upper House made little effort to investigate the scandal, perhaps for fear of damaging its public image and that of its highest official. Leaders of the governing coalition, concerned about a tight parliamentary schedule and key upcoming elections, reportedly tried to put off the day of reckoning.

Compared with the Lower House, the Upper House projects an image of "cleanliness" -- a quality that lends itself to added prestige and honor. With public confidence in politics at its nadir, though, it will take a long time before this "chamber of good sense" can restore that reputation.

The immediate challenge is to unravel the scandal. The resignation is not an alternative to parliamentary investigation. If nothing is done to find out the truth, or if half-way measures are taken to patch up the situation, public mistrust of politics could reach a point of no return. A coverup is a recipe for political suicide, particularly at a time of mounting calls for political accountability.

The scandal surfaced earlier this month when a weekly magazine reported that Mr. Inoue's government-paid secretary for policy affairs had accepted 64 million yen in funds from a public works contractor. The report said the contractor later asked the secretary to return 10 million yen -- which he did -- because the contract price turned out to be lower than promised.

Mr. Inoue told a different story: The secretary received only 10 million yen from the builder as a legitimate payment for fundraiser tickets. Later, however, the company demanded a full refund to make up for what it claimed was a loss on a project. The secretary, facing veiled threats to his family, returned the money without informing his boss.

The contractor offered another explanation: It handed the secretary 64 million yen in return for a public works job it had obtained. When the firm lost money because of a lower-than-expected contract price, it asked for the return of the whole sum. But it got back only 10 million yen.

It is a confusing story. Mr. Inoue's version raises disturbing questions: How is it that his secretary personally had to pay back as much as 10 million yen to the contractor? When did he learn that his aide had been blackmailed? (He says he learned it from the magazine report). And what was it that prompted the contractor to "extort" money?

Addressing these and other questions is primarily the responsibility of the Diet. Only by so doing can it begin to get at the root of the perennial corruption involving public works projects in Japan.

The day before his resignation, Mr. Inoue said the truth would be revealed in the course of prosecutors' investigations -- an oblique reference to the fact that his secretary has already filed a criminal complaint against the company's president. However, criminal responsibility is one thing; political and moral responsibility is another. The Diet should not, and cannot, sit on the sidelines.

Mr. Inoue has resigned to "draw a line" by taking political and moral responsibility. The Diet, which also has disgraced itself, must draw a line in its own way -- by probing the scandal as best it can and taking preventive measures. This is especially true in Mr. Inoue's case, given the high ethical standards expected of the Upper House president. Leaving everything to prosecutors is forsaking the parliamentary responsibility for self-reform.

Mr. Inoue's fall from grace is a wake-up call to the Upper House, which has lost some of the distinctive qualities that set it apart from the Lower House. One major problem is that it has become increasingly amenable to partisan influences. If it continues to act and think in much the same way as the Lower House, as if it were a clone of the lower chamber, then the upper chamber will end up losing its raison d'etre.

Now is the time for the Upper House to reassert its primary function of checking, balancing and complementing the Lower House. Reform along these lines is the only way to restore its honorable position as the "chamber of good sense" -- not just in name but in reality as well. The Inoue debacle is a fresh reminder that the Upper House is an integral part of political reform in this country.