A cylinder bore wall thermal insulator is installed in a groove-like coolant passage of a cylinder block having cylinder bores in an internal combustion engine to insulate the entire bore wall of all the cylinder bores or part of the bore wall of all the cylinder bores. The thermal insulator includes: a base member made of a synthetic resin and having a shape conforming to the shape of the groove-like coolant passage at the installation position of the thermal insulator; a cylinder bore wall thermal insulating member formed of a heat-expandable rubber and affixed to the inside of the base member; and a cylinder bore opposite wall contact member formed of a heat-expandable rubber and affixed to the outside of the base member. A thermal insulator can be obtained which is less likely to be displaced in the groove-like coolant passage, and is readily manufactured.