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; a water jacket spacer which is inserted in the water jacket portion; a passage through which the cooling medium in a portion of an inter-bore region is transferred to another portion of the inter-bore region, the inter-bore region being positioned in a vicinity of a boundary between the bore regions adjacent to each other; and a flow promotion device which increases a flow rate of the cooling medium flowing in the passage.