By Agence France-Presse
Construction workers found the weapons last month when they excavated the grounds of Tanashi Elementary School in the city of Nishitokyo in metropolitan Tokyo to build a new storehouse.
The weapons included 1,400 firearms, 1,200 swords, eight grenades and 302 bullets.
"We were totally surprised to be informed about the discovery," said Masaki Fujisawa, a city official. "The items are all rusted away and nothing can be used anymore."
"I suspect they were buried during or after the war but we will investigate the case," Fujisawa told AFP.
News reports said the weapons may have been discarded after Japan surrendered in 1945.
Unexploded bombs and weapons are still occasionally found in Japan, more than 70 years after the conflict ended.