A crankshaft support structure for an engine. An inner race forming a part of a main bearing is loosely fitted onto a bearing seat of a shaft. A direction of thrust is fixed by a flange of a timing sprocket which is press-fitted onto the shaft. The outer diameter R1 of the timing sprocket and the outer diameter R3 of the flange are respectively set so that they are smaller than the outer diameter R4 of the inner race. An outer race of the main bearing is press-fitted and fixed to the side of a left crankcase and a roller is integrated. When the shaft enters the inside of the roller, the timing sprocket and the flange pass through the inside of the roller, and the inner race is fitted to the roller. As a result, the inner diameter R2 of the inner race can be made smaller than the outer diameter R1 of the timing sprocket. Thus the main bearing supporting the crankshaft is miniaturized and lightened.