A blast treatment method for carrying out blast treatment of a treatment subject, comprising: a blasting preparation step of housing the treatment subject and a blasting explosive having fluidity in a container as well as attaching an initiation device to the container, the blasting explosive and the treatment subject being placed in the container in a manner that the blasting explosive covers the periphery of the treatment subject in the blasting preparation step, wherein the container includes a main body portion where an opening portion opening upward is formed, and a lid portion having a shape that covers at least a part of the opening portion, the blasting explosive and the treatment subject being housed in the main body portion of the container; a housing step of housing the container, the blasting explosive, and the treatment subject in a chamber; a decompression step of decompressing an inside of the chamber in a state where the chamber is sealed after the housing step; and a blasting step of initiating the blasting explosive by the initiation device and blasting the treatment subject by the blasting explosive, characterized in that the blasting preparation step further includes the steps of: providing the container with a gas vent portion that regulates escape of the blasting explosive from the container while permitting escape of gases in the container; hermetically sealing parts except the gas vent portion of the container, and mounting the lid portion on a top surface of the blasting explosive housed in the main body portion in a manner that the lid portion falls following a fall of the top surface of the blasting explosive, and wherein in the decompression step, the inside of the chamber is decompressed, an inside of the container is decompressed through the gas vent portion while the inside of the chamber is decompressed,while accompanying the fall of the top surface of the blasting explosive and the lid portion, wherein the blasting preparation step further includes a step of forming a gap with a dimension that regulates passage of the blasting explosive and permits escape of the gases between an outer surface of the lid portion and an inner surface of the main body portion enclosing the opening portion while mounting the lid portion on the blasting explosive in the main body portion, and in the decompression step, the gases in the container are discharged to the outside of the container through the gap functioning as the gas vent portion while the inside of the chamber is decompressed. a blasting preparation step of housing the treatment subject and a blasting explosive having fluidity in a container as well as attaching an initiation device to the container, the blasting explosive and the treatment subject being placed in the container in a manner that the blasting explosive covers the periphery of the treatment subject in the blasting preparation step, wherein the container includes a main body portion where an opening portion opening upward is formed, and a lid portion having a shape that covers at least a part of the opening portion, the blasting explosive and the treatment subject being housed in the main body portion of the container; a housing step of housing the container, the blasting explosive, and the treatment subject in a chamber; a decompression step of decompressing an inside of the chamber in a state where the chamber is sealed after the housing step; and a blasting step of initiating the blasting explosive by the initiation device and blasting the treatment subject by the blasting explosive, characterized in that the blasting preparation step further includes the steps of: providing the container with a gas vent portion that regulates escape of the blasting explosive from the container while permitting escape of gases in the container; hermetically sealing parts except the gas vent portion of the container, and mounting the lid portion on a top surface of the blasting explosive housed in the main body portion in a manner that the lid portion falls following a fall of the top surface of the blasting explosive, and wherein in the decompression step, the inside of the chamber is decompressed, an inside of the container is decompressed through the gas vent portion while the inside of the chamber is decompressed,while accompanying the fall of the top surface of the blasting explosive and the lid portion, wherein the blasting preparation step further includes a step of forming a gap with a dimension that regulates passage of the blasting explosive and permits escape of the gases between an outer surface of the lid portion and an inner surface of the main body portion enclosing the opening portion while mounting the lid portion on the blasting explosive in the main body portion, and in the decompression step, the gases in the container are discharged to the outside of the container through the gap functioning as the gas vent portion while the inside of the chamber is decompressed.