A cylinder bore wall thermal insulator (20, 40, 56) suitable for being set in a groove-like cooling water channel of a cylinder block of an internal combustion engine including cylinder bores to insulate a bore wall in a one-side half of bore walls of all the cylinder bores, the thermal insulator (20, 40, 56) comprising: one or more rubber sections (22, 42, 51) in contact with a wall surface on the cylinder bore side of the groove-like cooling water channel to cover the wall surface on the cylinder bore side of the groove-like cooling water channel; a base section (21, 41, 54) having a shape conforming to a shape of the one-side half of the groove-like cooling water channel, the one or more rubber sections (22, 42,51) or one or more members to which the one or more rubber sections (22, 42, 51) are fixed being fixed to the base section (21, 41, 54); and one or more elastic members (23, 43, 59) for urging the entire one or more rubber sections (22, 42, 51) to be pressed from a rear surface side toward the wall surface on the cylinder bore side of the groove-like cooling water channel, wherein the thermal insulator (20, 40, 56) includes a vertical wall (28, 28b) on a near side of a boundary (30, 30a, 30b, 30c) of each bore section (29, 29a1, 29a2, 29b1, 29b2) of the base section (21, 41, 54), the vertical wall (28, 28b) being in a middle and lower part of the groove-like cooling water channel in a flowing direction of cooling water, a setting position of the vertical wall (28, 28b) is a position where the ratio (x/y) of the length (x) of each bore section (29, 29a1, 29a2, 29b1, 29b2) of the base section (21, 41, 54) from one end of each bore section of the base section to the setting position of the vertical wall (28, 28b) to the length (y) of each bore section (29, 29a1, 29a2, 29b1, 29b2) of the base section (21, 41, 54) from one end of each bore section of the base section to the other end of each bore section of the base section is 0.75 to 0.9. the thermal insulator (20, 40, 56) comprising: one or more rubber sections (22, 42, 51) in contact with a wall surface on the cylinder bore side of the groove-like cooling water channel to cover the wall surface on the cylinder bore side of the groove-like cooling water channel; a base section (21, 41, 54) having a shape conforming to a shape of the one-side half of the groove-like cooling water channel, the one or more rubber sections (22, 42,51) or one or more members to which the one or more rubber sections (22, 42, 51) are fixed being fixed to the base section (21, 41, 54); and one or more elastic members (23, 43, 59) for urging the entire one or more rubber sections (22, 42, 51) to be pressed from a rear surface side toward the wall surface on the cylinder bore side of the groove-like cooling water channel, wherein the thermal insulator (20, 40, 56) includes a vertical wall (28, 28b) on a near side of a boundary (30, 30a, 30b, 30c) of each bore section (29, 29a1, 29a2, 29b1, 29b2) of the base section (21, 41, 54), the vertical wall (28, 28b) being in a middle and lower part of the groove-like cooling water channel in a flowing direction of cooling water, a setting position of the vertical wall (28, 28b) is a position where the ratio (x/y) of the length (x) of each bore section (29, 29a1, 29a2, 29b1, 29b2) of the base section (21, 41, 54) from one end of each bore section of the base section to the setting position of the vertical wall (28, 28b) to the length (y) of each bore section (29, 29a1, 29a2, 29b1, 29b2) of the base section (21, 41, 54) from one end of each bore section of the base section to the other end of each bore section of the base section is 0.75 to 0.9.