To precisely position a workpiece to a main spindle of a machine tool in a device for positioning an exchange material such as a workpiece to the main spindle of a metal processing machine such as a machine tool and an electric discharge machine via a jig plate.

In this device for positioning a workpiece (W) to a main spindle (2) via a jig plate (3), a central positioner (4) is arranged approximately concentrically to a central axis core (A) of the main spindle (2), and an eccentric positioner (5) is arranged at a position separated from the central axis core (A) in a radial direction. The central positioner (4) is configured to restrain an approximately entire periphery of a central hole (11) of the jig plate (3) in a radial direction to a central plug (21) projecting in an axial core direction from a supporting face (S) of the main spindle (2). The eccentric positioner (5) is configured to be freely switched between a release condition to form a fitting interval between the eccentric plug (61) projecting from the supporting face (S) in the axial core direction and an eccentric hole (51) of the jig plate (3), and a lock condition for restraining the eccentric hole (51) to the eccentric plug (61) in the peripheral direction of the central axial core (A) but not restraining it in a radial direction.