A heat retention material cartridge fixed to a base member of a heat retention tool of a cylinder bore's bore wall for heat retention of the cylinder bore wall, which includes a thermosensitive expansion rubber, a back-side pressing member, a rectangular opening, a front-side abutting plate cooperative with the back-side pressing member for sandwiching an outer edge portion of the thermosensitive expansion rubber, and an elastic member attached member to which an elastic member is attached. A convex portion being convex toward the side of the back-side pressing member is formed in a portion of the front-side abutting plate where the outer end is not sandwiched by bendable portions. And, the outer edge portion of the thermosensitive expansion rubber is formed in such a manner that the outer end of the thermosensitive expansion rubber is positioned inside an apex of the convex portion.