An outer stator is provided in a reciprocating engine together with an inner stator and a mover, that has a cylindrical shape and is placed around the mover coaxially with an axis of the mover, and that forms a magnetic path between itself and the inner stator via a field magnet of the mover, the outer stator includes an aperture portion that is formed in an inner circumferential surface thereof that faces the mover; an internal space that opens via the aperture portion, and in which a wound wire coil is placed; and undercut portions that, in a cross section including the axis, are provided at both end positions along the axis, and that are recessed from the inner circumferential surface towards the outer side in a radial direction.