The country's top five trading houses have announced more than ¥570 billion in asset writedowns, or about half the net income the firms expect to report for last year. And that's probably not the end of it.

"It'd be no surprise" to see Mitsui & Co. post as much as ¥100 billion in additional impairments when the second-largest trader reports its earnings next week, adding to the ¥48 billion booked in the prior quarter, said Daiwa Securities Group Inc. analyst Jiro Iokibe. In the quarter that ended March 31, rival Itochu Corp. may book ¥60 billion of writedowns, Iokibe said. Expect similar from Mitsubishi Corp., said Jefferies Group LLC analyst Thanh Ha Pham.

"There are still assets that the companies bought at high prices and that will need to drop" in value, said Iokibe, who in October put the maximum impairment risk at the top five at ¥1.5 trillion. At the time, less than ¥240 billion in writedowns had been announced.