Watakushi ga aishita kuchibiru(2000)

Makoto, a building cleaner, had always longed to become a woman. He was in love with Takeo, a man who frequently worked late at the building. However, upon learning that Takeo was married, Makoto drank heavily in despair and got into a fight with yakuza; just as he was being beaten within an inch of his life, a mysterious old man came to his rescue. The old man offered to grant Makoto's wish to become a woman. Makoto followed the old man's instructions and burned the talisman he had been given; the next morning, he had transformed into a woman named Mako. Mako then began working as a temporary employee at Takeo's company...