The use of natural enemies instead of chemical pesticides has been increasing worldwide. Although non-native natural enemies are beneficial and commonly used as biological control agents, the risks of non-target effects caused by their release are of growing concern. Phytoseiid mites are a very common choice as introduced biological control agents. In Japan, 8 species of non-native phytoseiid mites, one of which is the same as the native type but originates from a non-native population, have been introduced for crop pest control. There is concern about the influence of the introduced phytoseiid mites on native phytoseiid mites that occupy a similar habitat, but very few risk assessments of these phytoseiid mites have been carried out in Japan. In this paper, we estimate the risks of the non-target effect caused by the release of these introduced non-native phytoseiid mites by using the risk indexes proposed by Mochizuki (2010), and briefly present the results of monitoring the species considered at risk from the release of non-natives.