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 to cover a periphery of the treatment subject; and detonating the explosive from respective end portions of a specific direction toward a predetermined cutting position of the treatment subject between the respective end portions so that detonation waves of the explosive advancing from respective sides of the cutting position collide in the cutting position, 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.