Yashima received her first internaitonal medal at the 1994 World Junior Championships in Athletics, where she placed third in the women's 1500 metres closely behind Anita Weyermann and Marta Domínguez. She entered the senior ranks the following year and led the Japanese team to victory at the 1995 Asian Cross Country Championships, with Yashima and her team mates Michiko Shimizu and Yasuko Kimura sweeping the podium at the event on home turf in Chiba. This performance earned her selection for the 1995 IAAF World Cross Country Championships, where she finished 69th overall.