PURPOSE: To evolve a very small amount of carbonic acid gas for a long time, to attract and to catch mosquitoes simply and effectively, by processing a powder mixture consisting of a carbonate or bicarbonate and a deliquescent acidic salt with water in air, etc. into a wet state.

CONSTITUTION: Mixed powder consisting of 1mol carbonate or bicarbonate and 0.3W1mol deliquescent acidic salt (e.g., calcium chloride dihydrate) is processed with water in air or a very small amount of water drops fed, so that carbonic acid gas is evolved little by little for a long period. For example, the mixed powder in a wet state is put in an insect catching box whose inner side is coated with an adhesive, carbonic acid gas is evolved, mosquitoes are attracted and killed. Suitable especially for portable use, for using mainly it as an outdoor means to catch mosquitoes.