In any case, the Kuril Islands were a point in the San Francisco Peace treaty, in which it says that Japan has to renounce the islands + Sakhalin, etc. The issue arrives from the fact that Japan claims that the islands they claim aren't part of the Kuril Islands as stipulated in the Peace treaty.
It is more accurate to say the Japanese government NOW claims that, the Japanese government, PM and Foreign Ministry at the time clearly beleived the agreement to incude renunciation of the South Kurils now in "dispute", they testified to that in own parliament at time. Now Japanese 'conservatives' who hold positive views of fascist era dislike the SF agreement in general, but this doesn't mean Japanese government policy did not renounce these islands in treaty form. The link I posted is very detailed with related facts about this. https://www.japantimes.co.jp/opinion/2005/03/24/opinion/northern-territories-dispute-highlights-flawed-diplomacy/