A blast treatment method for blasting a treatment subject having an outer shell and a chemical agent charged into an interior of the outer shell includes the steps of: disposing an explosive on the outer side of the outer shell such that a difference occurs in a detonation pressure of the explosive, that acts on the outer shell from an outer side, between one side and another side of a predetermined cutting position of the outer shell and the outer shell is cut by a shearing force generated as a result of the difference in the detonation pressure; and detonating the explosive, wherein, in the step of detonating the explosive, the chemical agent is exposed by cutting the outer shell in the cutting position through detonation of the explosive, and the chemical agent is decomposed using the detonation.