A heating coil includes a pair of linear lead portions adapted to be connected to a power supply, and a ring-shaped or coil-shaped head portion having end portions connected to the lead portions respectively. A flow path through which a cooling medium flows is formed inside the lead portions and the head portion. An outer circumferential surface of the head portion has a circular or elliptical cross section and is configured to face a heating target portion of an inner circumferential surface of a work extending, the heating target portion extending in a circumferential direction of the work. Each of the lead portions has a straight side portion in a plane intersecting a longitudinal direction of the lead portions, the straight side portions of the lead portions being arranged in proximity to each other.