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, comprising the steps of: disposing an inside explosive to cover a periphery of the treatment subject; disposing an outside explosive having a higher detonation velocity than the inside explosive on an outer periphery of the inside explosive to extend from one end portion to another end portion of the inside explosive in a specific direction; characterized by connecting an initiation portion to the outside explosive, the initiation portion being for detonating the outside explosive from both end portions of the outside explosive in the specific direction simultaneously; wherein, in the step of disposing the outside explosive, the outside explosive is disposed to cover an entire periphery of the inside explosive in an orthogonal plane to the specific direction within a range extending to either side of a predetermined cutting position of the treatment subject in the specific direction, the blast treatment method further comprises the steps of: detonating the outside explosive by means of the initiation portion from respective end portions of the outside explosive in the specific direction toward the cutting position of the treatment subject between the respective end portions so that detonation waves of the outside explosive advancing from respective sides of the cutting position collide in the cutting position; detonating the inside explosive through detonation of the outside explosive from respective end portions of the inside explosive in the specific direction toward the cutting position of the treatment subject between the respective end portions of the inside explosive so that detonation waves of the inside explosive advancing from respective sides of the cutting position collide in the cutting position, wherein, in the steps of detonating the outside explosive and the inside explosive, the chemical agent is exposed by cutting the outer shell in the cutting position through detonations of the outside explosive and the inside explosive, and the chemical agent is decomposed using the detonations. disposing an inside explosive to cover a periphery of the treatment subject; disposing an outside explosive having a higher detonation velocity than the inside explosive on an outer periphery of the inside explosive to extend from one end portion to another end portion of the inside explosive in a specific direction; characterized by connecting an initiation portion to the outside explosive, the initiation portion being for detonating the outside explosive from both end portions of the outside explosive in the specific direction simultaneously; wherein, in the step of disposing the outside explosive, the outside explosive is disposed to cover an entire periphery of the inside explosive in an orthogonal plane to the specific direction within a range extending to either side of a predetermined cutting position of the treatment subject in the specific direction, the blast treatment method further comprises the steps of: detonating the outside explosive by means of the initiation portion from respective end portions of the outside explosive in the specific direction toward the cutting position of the treatment subject between the respective end portions so that detonation waves of the outside explosive advancing from respective sides of the cutting position collide in the cutting position; detonating the inside explosive through detonation of the outside explosive from respective end portions of the inside explosive in the specific direction toward the cutting position of the treatment subject between the respective end portions of the inside explosive so that detonation waves of the inside explosive advancing from respective sides of the cutting position collide in the cutting position, wherein, in the steps of detonating the outside explosive and the inside explosive, the chemical agent is exposed by cutting the outer shell in the cutting position through detonations of the outside explosive and the inside explosive, and the chemical agent is decomposed using the detonations. detonating the outside explosive by means of the initiation portion from respective end portions of the outside explosive in the specific direction toward the cutting position of the treatment subject between the respective end portions so that detonation waves of the outside explosive advancing from respective sides of the cutting position collide in the cutting position; detonating the inside explosive through detonation of the outside explosive from respective end portions of the inside explosive in the specific direction toward the cutting position of the treatment subject between the respective end portions of the inside explosive so that detonation waves of the inside explosive advancing from respective sides of the cutting position collide in the cutting position, wherein, in the steps of detonating the outside explosive and the inside explosive, the chemical agent is exposed by cutting the outer shell in the cutting position through detonations of the outside explosive and the inside explosive, and the chemical agent is decomposed using the detonations.