A cylinder bore wall thermal insulator includes a base member made of synthetic resin and having a shape conforming to a shape of the groove-like cooling water channel in a setting position of the thermal insulator, an opening for heat-sensitive expanding rubber swelling for heat-sensitive expanding rubber disposed on a rear surface side to pass through a base member during heat-sensitive expansion being formed in a position opposed to an insulating part of a cylinder bore wall, heat-sensitive expanding rubber disposed on the rear surface side of the base member and covering the opening for heat-sensitive expanding rubber swelling, and a rear-surface metal plate covering the rear surface side of the heat-sensitive expanding rubber, fixed to the base member, and holding an outer edge portion of the heat-sensitive expanding rubber between the rear-surface metal plate and the base member to thereby fix the heat-sensitive expanding rubber to the base member. An urging member for urging the heat-sensitive expanding rubber after the heat-sensitive expansion toward the cylinder bore wall is attached to the rear-surface metal plate. According to the present invention, it is possible to provide a thermal insulator that has high adhesion to a wall surface on a cylinder bore side of a groove-like cooling water channel, less easily causes positional deviation in the groove-like cooling water channel, and is easily manufactured.