Zessho (1975)

Junkichi is the son of a rich landlord, while Koyuki is the daughter of a poor lumber worker. The mismatched pair soon finds love, and against his father's wishes, Junkichi decides to run away with Koyuki to start a new life. But when Junkichi is called off to war, they make a pledge to sing the song of engagement everyday until they are able to reunite