Tomita, who is 20 years old, is currently in critical condition after having received nearly two dozen stab wounds on the neck and chest.
According to police, Iwazaki allegedly admitted that he had stabbed Tomita due to her returning a gift he had sent her, without clearly explaining the reason why.
The @ JAM 2016 event took place at Zepp DiverCity Tokyo on Saturday and Sunday.
He had allegedly been waiting for her at the station to ask her about the gift.