A load due to combustion pressure is applied to each of the crank pins via a connecting rod in a direction from the axis of a piston pin to the axis of the crank pin. Each of the crank arms includes a recess in a surface adjacent to the journal, in a region inward of a peripheral region along the edge of the surface. The recess extends along the peripheral region, and is asymmetric with respect to a crank arm centerline connecting the axis of each of the crank pins to the axis of each of the journals. Each of the crank arms has a maximum flexural rigidity at a point of time when the load onto each of the crank pins due to the combustion pressure reaches a maximum. The crankshaft has a reduced weight and an increased torsional rigidity in combination with an increased flexural rigidity.