Nineteen 10,000 yen notes were placed in the mailboxes of four homes in the town of Fukiage, Saitama Prefecture, between Friday night and Saturday morning, local police said.
There was no letter attached to the notes, which were not bundled together, and the cash is being treated as lost property.
Two of the four homes received 60,000 yen, while 30,000 yen and 40,000 yen were placed in the mailboxes of the other two homes, according to police.
The first household to find the money did so at around 11 p.m. Friday, and investigators said they believe all the money was distributed sometime Friday night.