"NTT Communications has informed us about this. We would like them to conduct a thorough investigation," Kono said at a press conference, as quoted by the Kyodo news agency.
Speaking at the news conference, Kono stressed the importance of businesses sticking to highest cybersecurity rules.
"We want to make sure that they comply with all the rules established by the Defense Ministry to avoid leaks," the minister said in a reference to the recent increase in cyberattacks on companies involved in the defence industry sector.
On Thursday, Kyodo reported, citing sources, that NTT Communications was subjected to a cyberattack earlier in May, as a result of which data concerning communications networks of the Japan Self-Defence Forces could have been leaked.
In total the hackers received unauthorised access to over 100 data files, all of them allegedly linked to firms linked to the ministry. According to Kyodo, the compromised data does not appear to be secret, but leaks still may undermine the communications network.
Earlier in May, media reported that hackers during an attack on Mitsubishi Electric in January might have obtained data concerning research on a supersonic glide missile.