A method of manufacturing a mechanical part of the present invention includes a first process of forming, by performing a folding processing to an end portion of the material, a portion to be processed having a structure, in which a plurality of layers respectively having a thickness corresponding to a plate thickness of a material overlap each other, in the material such that a plate thickness direction of the layer is orthogonal to a plate thickness direction of the material; and a second process of changing, by performing a forging processing to the portion to be processed, a shape of the portion to be processed to a target shape while press-welding the layers of the portion to be processed to each other by plastic deformation.