To provide a holder which has a gripping force suitable for machining and reduces distortions of a workpiece occurring upon holding it to enable a highly accurate machining, and a holding method for the workpiece.

The holder 1 is used for fitting a workpiece 7 on a machine tool. The holder 1 comprises a cylindrical collet 8 on part of which cut-splits are formed, a working cylinder 15 reciprocating in the direction of the axis (A) of the collet 8, and a flexible ring 19, which is set between the collet 8 and the working cylinder 15 with a prescribed gap (a) formed between the cylinder 15 and the ring 19 in perpendicular to the axis (A), and deforms in accordance with the surface shape of the workpiece 7 upon reciprocating in response to the reciprocation of the actuating cylinder 15.