A cooling structure for uniformly cooling a bore wall of a cylinder block using a cooling medium, the bore wall surrounding plural bore regions, includes a water jacket portion which is provided so as to surround an entire outer periphery of the bore wall, and which is supplied with the cooling medium; and a water jacket spacer which is inserted in the water jacket portion such that a space is provided between the bore wall and the water jacket spacer. The cylinder block includes an inter-bore region which is positioned in a vicinity of a boundary between the bore regions adjacent to each other, and a region other than the inter-bore region; and a cross sectional area of the space between the bore wall and the water jacket spacer in the inter-bore region is smaller than a cross sectional area of the space between the bore wall and the water jacket spacer in the region other than the inter-bore region.