1. A method for inspecting a crankshaft, comprising: a first step of disposing an optical three-dimensional shape measurement device to face a crankshaft in a direction perpendicular to a rotational center axis of the crankshaft, wherein the three-dimensional shape measurement device is configured to measure a three-dimensional shape of a measurement object by projecting and receiving light to and from the measurement object; a second step of measuring a surface shape of the crankshaft by the three-dimensional shape measurement device disposed in the first step to acquire three-dimensional point cloud data of a surface of the crankshaft over an entire length of a measurement object region of the crankshaft; a third step of dividing the three-dimensional point cloud data acquired in the second step to create a plurality of subregion three-dimensional point cloud data, each of the subregion three-dimensional point cloud data respectively corresponding to each of a plurality of subregions of the crankshaft along a direction parallel to the rotational center axis of the crankshaft; a fourth step of translating and rotating each of the subregion three-dimensional point cloud data created in the third step to superpose each of the subregion three-dimensional point cloud data on a surface shape model of the crankshaft prepared in advance based on a design specification of the crankshaft, such that a distance between each of the subregion three-dimensional point cloud data and the surface shape model becomes minimum; and a fifth step of calculating a distance between each of the subregion three-dimensional point cloud data after being superposed in the fourth step and the surface shape model, and detecting a partial defect such as an underfill of the crankshaft based on the distance calculated.