The writer observed the relation of root pests to black rot of sweet potato root-tubers in the field. The results are summarized as follows:- 1. Black rot fungus invades the root-tuber mostly from the eaten cavity caused by various root pests: for example, Cylas formicarius, Colasposoma oberthuri etc., which are the common pests of sweet potato in Okinawa-honto. Consequently, the extermination of these pests in the field is a method of protection from black rot on sweet potato root-tuber. 2. The longer the time elapsed after the formation of eaten cavity, the more difficult is the infection by the pathogene.