Around 50 people complained of symptoms related to heat exhaustion Saturday during a rock concert at Makuhari Messe convention center in Chiba Prefecture, police said.
At least 13 were taken to hospital but none were found to be suffering from any serious symptoms or injuries, the police said.
Many felt unwell and left the venue during the concert by Japanese rock band One OK Rock on Saturday evening.
A teenage female fan said the site was crowded and hot.
"The performance was interrupted after someone screamed and people at the back of the venue became confused," a woman in her 20s said.