A woman found a 1-cm-long fingertip in a rice ball bought at a Lawson Inc. convenience store in Sendai, Miyagi Prefecture.
The manufacturer apparently failed to recover the tip after a plant worker cut a finger, the Sendai municipal government said Saturday.
The woman reported to police around 7 p.m. Friday that she had found a piece of flesh in the rice ball after biting into it and sensing something unusual in her mouth.
A doctor later confirmed the object was a fingertip.
The city ordered Nihon Fresh Delica's Sendai plant, which produced the rice ball, to suspend its business on Sunday and recall some 64,000 rice balls from the market.
The company is an affiliate of Nippon Flour Mills Co., the oldest and second-largest flour producer in Japan, according to the government.
Nihon Fresh Delica told police that one of its factory workers had been injured by a pressing machine. The company immediately discarded dozens of rice balls after the accident, but the right-hand index fingertip, around 1.2 cm long, apparently got into a rice ball that was shipped.
The Sendai factory delivers rice balls to 228 Lawson stores in the Tohoku region of northeastern Japan.