Hey Misa J san !! A favorite name of mine !! (as you know)
That must be the 'Misato'--well Misa...窶ーツス窶堙??堋ゥ ...I always get the name a little mixed up, but from the Gunma area, going into Matsumoto, and then on to Shiojiri, then over the kougen to Okaya, and then around the lake into Suwa, that would be the one.
Otherwise, I can't think of a long tunnel from that area into the Suwa area. You'd have to go up to the Venus Skyline, Utsukushigahara area, and then down. That tunnel is still here, and rather often closes one lane or both, for repairs--and it's a toll tunnel.
That movie is nice. I hope you can find it !!