Provided is a method for cleaning the inside of a pressure tight container for a blasting treatment, wherein the inside of a pressure tight container can be cleaned for a short period of time after a blasting treatment. The method is comprised of a setting process wherein a cleaning blast (1) capable of cracking a part of a residual substance remaining on the inside of a pressure tight container (10), and a solid separating material (2) which collides with a substance bonded to an inner wall surface (32a) of the pressure tight container (10), to separate the bonded substance from the inner wall surface (32a) of the pressure tight container (10), are set on the inside of the pressure tight container (10); and a separation process wherein the cleaning blast (1) blasts on the inside of the pressure tight container (10), to crack a part of the residual substance remaining on the inside of the pressure tight container (10), and the separating material (2) is divided into a plurality of granular elements by the blast, so that the granular elements are spread, and collide with the bonded substance to remove the bonded substance.