A cooling structure includes a block-side water jacket formed in a cylinder block, a head-side water jacket formed in a cylinder head, an introducing portion which introduces a cooling liquid from an end of the cylinder block to the block-side water jacket, a discharging portion which discharges a cooling liquid from the other end of the cylinder block to the head-side water jacket, and a spacer member accommodated in the block-side water jacket, and including a peripheral wall which forms an exhaust-side passage and an intake-side passage between a cylinder bore wall and the peripheral wall. The spacer member includes a distribution adjustment mechanism which distributes a cooling liquid introduced to the block-side water jacket between the exhaust-side passage and the intake-side passage.