Cooling structure for internal combustion engine

A height (H) of a spacer (14) which is arranged inside a water jacket formed to surround peripheries of cylinder bores of a cylinder block of an internal combustion engine is made to be lower by a step (t) in an intermediate portion thereof in a cylinder row line (L1) direction. Consequently, among end-portion cylinder bores having a lower temperature and being situated at opposite end portions in the cylinder row line (L1) direction and intermediate cylinder bores being other than the end-portion cylinder bores and having a higher temperature, a cooling performance of the intermediate cylinder bores is higher than that of the end-portion cylinder bores. Therefore, all the temperatures of the respective cylinder bores can be made uniform. A similar operation effect can be obtained alternatively by making a thickness (T1) of the spacer (14) thinner in portions facing the intermediate cylinder bores and thicker in portions facing the end-portion cylinder bores.