Police in 2003 investigated a record 145 suspected violations of a law banning illegal access to computer networks, the National Police Agency said Thursday.
The NPA said the number of such cases was the highest since the law took effect in February 2000, up by 40 from a year earlier.
Many of the cases involved stolen passwords, often guessed at based on personal identification used at Internet auctions. After stealing passwords, thieves actually purchased products on the auctions.
"We recommend computer users use more complex passwords or change them regularly," an NPA official said.
Police took action against 76 people in 2003, mainly by arresting them or sending their cases to prosecutors. This was up seven, or 10 percent, from 2002. People in their 20s accounted for the largest single group, at 26, followed by those in their 30s at 24. Sixteen were teenagers.
In one case last March, the Metropolitan Police Department arrested a man in his 30s after he withdrew about 16 million yen from the bank account of an online banking user after the man allegedly stole the user's password and other information by installing a special "key-logger" program in an Internet cafe.
Last June, Tokyo police sent papers to prosecutors on a high school student in Hamamatsu, Shizuoka Prefecture, who allegedly wrote anti-Iraq war messages on the Web sites of 23 countries after illegally accessing them.