In 1992, Koiwai finished 8th in her debut at the World Junior Championships. She won gold at the event in 1993. Coached by Machiko Yamada, who had trained Midori Ito, Koiwai was routinely landing the triple axel jump by 1994. However, she faced several chronic injuries on her right foot, which severely impeded her efforts. In 1996, she made her only appearance at the senior World Championships, finishing 16th. In 1997, she won gold at the Winter Universiade and then retired from competition.