Abstract: The present invention relates to a method of cultivating, in medium, Steinernema kushidai (hereinafter simply called kushidai) which is one type of Nematoda. Kushidai has a strong insecticidal ability to kill larvae of coliopterous insects, and cultivation of large quantities of kushidai having the strong insecticidal ability requires conditions different from the techniques of cultivating other nematodes parasitic on insects.Kushidai is cultivated in a medium including asteroid. This increases the insecticidal activity with respect to insects harmful to plants such as crops. The medium including asteroid may be adsorbed to a plurality of supports, and then the supports may be stacked with gaps thereamong to form a layer. This enables kushidai to be cultivated efficiently. Further, a liquid medium including asteroid may be prepared, with cultivation carried out while agitating the medium and supplying oxygen thereto.