Concentrations of Cs-134 and Cs-137 in 205 wild mushroom samples collected in Yamanashi Prefecture from September 2011 to October 2013 were measured. In all of wild mushroom samples, the concentration of Cs-137 was higher than that of Cs-134; in some samples, Cs-137 was detected while no Cs-134 was detected. Even when the concentrations of Cs-134 and Cs-137 were compared in consideration of their half-lives on the assumption that these radioactive cesium elements were brought due to the Fukushima Daiichi Nuclear Power Plant (NPP) accident, the concentration of Cs-137 was still significantly higher than that of Cs- 134. As the amounts of Cs-134 and Cs-137 released from the Fukushima Daiichi NPP were almost equivalent, the finding indicates that the collected wild mushrooms contained Cs-137 not derived from the Fukushima Daiichi NPP accident. In fact, the maximum concentration of Cs-137 not derived from the NPP accident was more than 200 Bq/kg in a sample.