A cylinder bore wall thermal insulator includes one or more rubber sections in contact with a wall surface on the cylinder bore side of a groove-like cooling water channel, a base section having a shape conforming to a shape of a one-side half of the groove-like cooling water channel, the one or more rubber sections or one or more members to which the one or more rubber sections are fixed being fixed to the base section, and one or more elastic members for urging the entire one or more rubber sections to be pressed from a rear surface side toward the wall surface on the cylinder bore side of the groove-like cooling water channel. The thermal insulator includes a vertical wall near a boundary of each bore section of the base section and on a near side of the boundary of each bore section of the base section.