1) There is no Japanese Navy. The naval force is called the Maritime Self Defense Forces.

2) The MSDF did not sink the North Korean ship. It was the Japan Coast Guard.

Although Japan has one of the largest defense budgets in the world, its military does not have an offensive capability. Japan has no bombers. It cannot support an expeditionary force necessary to invade or occupy another country. It has no ballistic missiles or nuclear capability. Japan will not even be eligible to receive the Raptor fighter, and will have to make due for the time being with its aging fleet of interceptors.

Probably the most controversial new military capability Japan has is its acquisition of the Aegis ballistic missile defense system. But this is a defensive capability deployed in response to what could really be called a clear and present danger, the threat of nuclear attack by North Korea.

I suppose what people are most concerned about is the movement to change Japan’s Constitution to allow the country to become a “normal” or “regular” country that can resort to war in certain situations.

Personally, I’m disappointed that the current Japanese government has chosen to do this. Why not continue to renounce war? Japan is already a leader in the fight to combat climate change. Why not be a leader in the effort to promote peace?

China’s current strategic thinking about the region over the coming five
to ten years assumes that the PRC itself will be stronger and more important,
that cultivating good ties with the ROK is fundamental to almost all potential
futures, that avoiding a major break with the United States will remain in
Beijing’s interest for more than a decade to come, and that the two major wild
cards are the future actions of both Japan and North Korea.

http://nbr.org/publications/asia_policy/AP3/AP3_NEASecRT.pdf

It might be that the new administration will take measures based on his view.

Who could have been responsible for it?

You can find more or less likely-minded people here:
http://www.jusfc.gov/members.asp
including Jay “I wrote the preface to Ozawa’s book, Nihon Kaizou keikaku” Rockefeller.