The reported fresh data leak may impede Tokyo's efforts to accelerate implementation of its missile defence programme after North Korea test-fired a volley of missiles last July.
The reports follow a police raid on Saturday of a naval college in western Japan over a leak of data earlier this year on the high-tech Aegis radar system, used by the United States on ships fitted with SM-3 ballistic missile interceptors.
One of Japan's Aegis ships is set to be fitted with SM-3 missile interceptors, similar to those operated by the United States, by the end of this year.
In addition to the data on the SM-3 interceptors, police found that confidential material was leaked regarding the ''Link 16'' information distribution system -- the latest system used in major US warships and fighter jets, Kyodo said.
Data on the interceptors and the information system are classified under a 1954 law on protecting secrets related to the Japan-US mutual defence agreement, Kyodo said.
Japanese investigators would neither deny nor confirm the reports.
US government officials were unnerved by the leak earlier in the year of sensitive information, which came just as Tokyo and Washington vowed to forge closer defence ties.
US Defence Secretary Robert Gates voiced his concerns when he met his Japanese counterpart Fumio Kyuma in Washington recently, and Kyuma vowed to take steps to prevent any recurrence of such leaks.
REUTERS SM VV1320