A crankshaft bearing structure of a multi-cylinder internal combustion engine, comprising: two cylinders adjacent to each other in which each piston moves in phase, a crankshaft, and a main bearing unit (#1 to #5) at each end portion of the crankshaft as well as at an inter-cylinder position of the crankshaft, characterized in that at a normal temperature of the internal combustion engine, a bearing clearance in the main bearing unit (#3) between the two cylinders is substantially equal to the bearing clearances in the other main bearing units (#1, #2, #4, #5), as the engine temperature becomes higher than the normal temperature, the bearing clearance in the main bearing unit (#3) between the two cylinders becomes larger than the bearing clearances in the other main bearing units (#1, #2, #4, #5), the internal combustion engine is a four-cylinder internal combustion engine comprising five main bearing units, and the bearing clearance of a No. 3 main bearing unit (#3) in a center is larger than the bearing clearances of the other main bearing units (#1, #2, #4, #5) at a time of high engine temperature, and the No. 3 main bearing unit (#3) is configured so that the bearing clearance of the No. 3 main bearing unit (#3) is smaller than the bearing clearances of the other main bearing units (#1, #2, #4, #5) when the internal combustion engine is cold, and the bearing clearance of the No. 3 main bearing unit (#3) is substantially equal to the bearing clearances of the other main bearing units (#1, #2, #4, #5) at a temperature of a normal range of the internal combustion engine after warm-up. two cylinders adjacent to each other in which each piston moves in phase, a crankshaft, and a main bearing unit (#1 to #5) at each end portion of the crankshaft as well as at an inter-cylinder position of the crankshaft, characterized in that at a normal temperature of the internal combustion engine, a bearing clearance in the main bearing unit (#3) between the two cylinders is substantially equal to the bearing clearances in the other main bearing units (#1, #2, #4, #5), as the engine temperature becomes higher than the normal temperature, the bearing clearance in the main bearing unit (#3) between the two cylinders becomes larger than the bearing clearances in the other main bearing units (#1, #2, #4, #5), the internal combustion engine is a four-cylinder internal combustion engine comprising five main bearing units, and the bearing clearance of a No. 3 main bearing unit (#3) in a center is larger than the bearing clearances of the other main bearing units (#1, #2, #4, #5) at a time of high engine temperature, and the No. 3 main bearing unit (#3) is configured so that the bearing clearance of the No. 3 main bearing unit (#3) is smaller than the bearing clearances of the other main bearing units (#1, #2, #4, #5) when the internal combustion engine is cold, and the bearing clearance of the No. 3 main bearing unit (#3) is substantially equal to the bearing clearances of the other main bearing units (#1, #2, #4, #5) at a temperature of a normal range of the internal combustion engine after warm-up. the bearing clearance of the No. 3 main bearing unit (#3) is smaller than the bearing clearances of the other main bearing units (#1, #2, #4, #5) when the internal combustion engine is cold, and the bearing clearance of the No. 3 main bearing unit (#3) is substantially equal to the bearing clearances of the other main bearing units (#1, #2, #4, #5) at a temperature of a normal range of the internal combustion engine after warm-up.