Japan, by nature, is very reticent about mentioning scenarios or methods of pre-emptive missile strikes against North Korean ballistic missile threats. North Korea likely assumes a pre-emptive strike would come in overwhelming nuclear force from US submarines or perhaps even from China (to hose down the likelihood of WWIII).
Unfortunately the possibility that Donald Trump might win the Presidential Election (on November 8, 2016) provides Japan with no assurance about what kind of deterrence a Trump Administration would exert against North Korea.
South Korea was more forthcoming in late May 2016 when it released news about a project to develop an SLBM for South Korea's future class of submarine - the 3,000-4,000 ton Jangbogo-3 (also known as KSS-III or KSS-3). Japan would also be considering submarine launched land attack cruise (and maybe ballistic) missiles. Japan may also desire parity with a South Korean submarine launched missile capability for its own sake. Japan and South Korea are neither enemies nor friends.