The weapons were found buried about two meters under the ground of the Tanashi Elementary School in the city of Nishitokyo, Tokyo Metropolis.
The discovery was made on July 27 by workers during evacuation work for constructing a building at the school, and the weapons have been safely removed from the ground, said the officials.
The weapons were believed to have belonged to the Japanese army and were probably discarded there after the World War Two ended in 1945, according to the officials.
It's rather rare for such a large amount of weapons to be found buried at residential areas, said local media reports.
The local government said it will discuss with the police how to properly deal with the weapons.