To provide a gripping tool which minimizes a decentering error and a tilting error of a machining device with respect to a rotating shaft, and achieves attachment thereto/detachment therefrom of a workpiece with high precision, without incurring increase of a machining lead time and degradation of an availability of the machining device, and to provide a gripping method and a machining device.

According to the structure of the gripper 4, a cylindrical tool body 4a of the gripper has a recessed guide portion 11 formed therein, which has a recessed tapered surface 11a expanding from a first opening edge 4b where a gripping hole 7 for holding the workpiece 1 is formed, toward a second opening edge 4c, and the first opening edge 4b and the second opening edge 4c have a plurality of proximal end slits and distal end slits formed therein such that the former are deviated from the latter in circumferential direction of the tool body 4a. Thus by expanded deformation of the recessed guide portion 11 with respect to a projective guide portion 8 at the time of mounting the workpiece, contractive deformation occurs at the gripping hole 7 formed in the first opening edge 4b where the distal end slits are formed. In this manner, aligning of the workpiece 1 (workpiece reference shaft C) located in the gripping hole 7 with respect to a main rotating spindle A is automatically achieved.