Various harmful insects exist in the land facilities and houses. An investigation research is widely done about the harmful insects. And an insecticide against them is variously developed, and a report about their effect is done. Harmful insects exist in a ship as well as on the land. However, a research report regarding them is a few. In Nagasaki University, we are getting rid of harmful insects in the training ship Kakuyo-maru in June or July every year. However, approximately 3 months later, an inhabitation number of a cockroach returned as it was before. Because 20 years elapsed after the Kakuyo-maru constructed, it offered a cochroach comfortable trace to exist in the ship. Then, we have executed a biannual cockroach extermination using bait medicine from 1997. We continue a state that there is hardly any cockroaches, i. e. cockroach index is less than 0.1 through the year. Consequently, it is thought that the using of bait medicine indicates a persistent effectively in a ship.