To protect against or exterminate insects carrying disease germs for plants such as termite which damage a building material or a tree by eating, a wood rotting bacteria which rots wood and further, a harmful insect such as cockroach by an effective action.

This method for exterminating insect carrying disease germs and wood rotting bacteria is to apply or spray a diatom as a single component or as a principal component, or applying or spray a controlling agent obtained through processing in such a way that it can be used by laying it in an effective spot, or laying it in such a spot to check the generation or the intrusion of the termite or the wood rotting bacteria, in every site of a building or under the floor which is susceptible of damage by the terminite or the wood rotting bacteria or has already been damaged.