A cooling structure of a multi-cylinder engine is provided, which includes a water jacket formed in a cylinder block to surround cylinder bores of cylinders arranged inline, a spacer, and a coolant inlet. The spacer includes openings at positions corresponding to inter-cylinder-bore portions and a rectifying part extending outwardly on a lower side of the openings. The rectifying part inclines continuously upwardly while extending in one of an exhaust- and intake-side section of the jacket from a first end side to a second end side that is the opposite side from the first end side in a cylinder line-up direction, extending on the second end side from the one of the exhaust- and intake-side sections to the other one of the exhaust- and intake-side sections, and then extending from the second end side to the first end side in the other one of the exhaust- and intake-side sections.