Hold it right there. I would not speak to the immigration and customs people in Japanese, unprompted, unless they ask you if you speak it. Then you can say "a little", or whatever in a sheepish, aw-shucks kind of way, and right away tell them why you speak it.

Letting them know that you speak Japanese but don't live in Japan is an invitation to the proctologist desk in the backroom. They are highly suspicious of foreigners who speak Japanese but don't live there. They find it hard to comprehend that somebody speaks Japanese but has never lived in Japan. They will assume that you are lying and that you are living in Japan on a tourist visa. A blank passport (like yours) is not going to appease them. It may actually arouse even further suspicions given that you may have "lost" or "misplaced" your other passport with your previous entries.

Also, I don't know how much you have studied Japanese, so I can't comment on your level of fluency, but my guess if your studies have been of less than 2-3 years of fairly intensive or immersive Japanese, you are probably not able to hold a meaningful conversation with the immigration officers besides "I like Japan, I have studied Japanese for x months/years, and I want to be a tourist in your country".