A method for cleaning the inside of a pressure tight container for blasting treatment, the method being performed after a blasting step in which an object to be blasted and a blasting explosive are set and blasted inside the pressure tight container, the method comprising: a setting step of setting a cleaning explosive different from the blasting explosive inside the pressure tight container after the blasting step, placing a solid separating material formed of a plurality of granular elements in a separation container different from the pressure tight container, and setting the separation container containing the solid separating material formed of a plurality of granular elements inside the pressure tight container such that the plurality of granular elements are placed around the cleaning explosive and the plurality of granular elements at least partially cover the cleaning explosive and thus at a position which allows the separating material to be scattered to an inner wall surface of the pressure tight container by explosion of the cleaning explosive and to collide with the inner wall surface of the pressure tight container, the separating material being capable of removing adhering substances adhering to the inner wall surface of the pressure tight container after the blasting step by colliding with the adhering substances; and a removing step of exploding the cleaning explosive inside the pressure tight container to partially decompose a residue of the object to be blasted remaining inside the pressure tight container after the blasting step, breaking the separating material into a plurality of granular elements, and causing the granular elements to scatter to and collide with different parts of the inner wall surface of the pressure tight container to remove the adhering substances. a setting step of setting a cleaning explosive different from the blasting explosive inside the pressure tight container after the blasting step, placing a solid separating material formed of a plurality of granular elements in a separation container different from the pressure tight container, and setting the separation container containing the solid separating material formed of a plurality of granular elements inside the pressure tight container such that the plurality of granular elements are placed around the cleaning explosive and the plurality of granular elements at least partially cover the cleaning explosive and thus at a position which allows the separating material to be scattered to an inner wall surface of the pressure tight container by explosion of the cleaning explosive and to collide with the inner wall surface of the pressure tight container, the separating material being capable of removing adhering substances adhering to the inner wall surface of the pressure tight container after the blasting step by colliding with the adhering substances; and a removing step of exploding the cleaning explosive inside the pressure tight container to partially decompose a residue of the object to be blasted remaining inside the pressure tight container after the blasting step, breaking the separating material into a plurality of granular elements, and causing the granular elements to scatter to and collide with different parts of the inner wall surface of the pressure tight container to remove the adhering substances.