Provided is a heat retention means for a cylinder bore wall, characterized in that: the heat retention means includes a bore wall heat retention part which is installed on each bore wall of each cylinder bore, and a support part to which the bore wall heat retention part is fixed and which has a shape that conforms to the shape of a grooved cooling water flow path at the installation position of the heat retention means; each bore wall heat retention part comprises a rubber member, a back surface pressing member, and an elastic member; a cooling water passage port for allowing cooling water onthe back surface side of a support part bore space to pass through to the inside is formed at at least one location on the upper part of the support part; the support part has a guide wall in the vicinity of the cooling water passage port, and has an inclined wall on the back surface side of support part bore spaces; and, in each bore wall heat retention part, only the center or near center in the arc direction is fixed to the support section. According to the present invention, it is possible to provide a heat retention tool which is in intimate contact with the side wall of the groove-shaped cooling water flow path on the cylinder bore side, can selectively thermally insulate a zone where heat retention is necessary, and has high cooling efficiency at the boundary of the bore wall of each cylinder bore and in the upper part in the vicinity thereof.