The Japan Aerospace Exploration Agency, or JAXA, says it has suffered multiple cyberattacks since last year into this year and some data may have been leaked.
Sources close to the matter say highly confidential documents may have been among those accessed.
JAXA says it was subjected to unauthorized external access last year and that it might have led to personal information of staff being leaked from its servers.
The space agency says since then it had been looking into the extent of damage and how its system was compromised, but cyberattacks on JAXA continued into this year.
The agency says an investigation is still underway regarding data that may have been leaked. It declined to provide details, citing security reasons.
It says information related to national security, such as the operation of rockets and satellites, is handled on a separate network and was not compromised in the attacks.
The sources say more than 5,000 data sets were stored in the server hacked last year, including personal information ranging from senior JAXA officials to temporary staff.
They say there is a possibility that hackers used such data to access highly confidential documents, which are protected under non-disclosure agreements between JAXA and companies or other organizations.
JAXA says the attacks have caused concern for related companies, organizations and citizens. It says it will continue looking into the matter and take measures to address further attacks.
Japanese Chief Cabinet Secretary Hayashi Yoshimasa told reporters that JAXA is conducting a detailed investigation with the help of specialized institutions, including the National center of Incident readiness and Strategy for Cybersecurity.
He said he wants the space agency to swiftly deal with the matter and take necessary measures. He said cyberspace is becoming more complicated and the government will work to step up security in that field.