A part positioner according to the present invention is used for fixing a part having a first pair of opposite portions extending along a mounting surface to the mounting surface by use of a clamping fixture. The part positioner includes a pressing member for pressing the part against the mounting surface with a clamping force of the clamping fixture. The pressing member includes a pressing portion (tapered projection) for pressing the first pair of opposite portions in opposite directions in a position apart from the clamping fixture, owing to a clamping operation of the clamping fixture.