A spacer fitted inside a water jacket of a cylinder block in an internal combustion engine is set so that a space formed between an inner peripheral surface of the spacer and an inner wall surface of the water jacket is smaller than a space formed between an outer peripheral surface of the spacer and an outer wall surface of the water jacket. Accordingly, even if a position of the spacer is shifted in a radial direction, the inner peripheral surface of the spacer comes into abutment on the inner wall surface of the water jacket at first. Thereby, the abutment of the outer peripheral surface of the spacer on the outer wall surface of the water jacket is prevented completely. Therefore, hitting sounds of pistons can be blocked by the space between the outer peripheral surface of the spacer and the outer wall surface of the water jacket.