Induction heating coil

An induction heating coil comprising:

• a coil configured to heat a treatment target by induction;
• a power supply configured to supply power to the coil;
• and
• a cooling medium passage that is configured to supply a cooling medium to the coil,
• wherein the coil is formed in the shape of an open circle that has one end and an other end,
• the cooling medium passage includes a power supply side passage formed in the power supply and a coil side passage formed in the coil,
• the coil side passage includes an inlet connected to the power supply side passage, and an outer circumferential surface and an inner circumferential surface arranged to extend along a circumferential direction of the coil,
• each of one end portion of the inner circumferential surface and one end portion of the outer circumferential surface are connected to the inlet, and
• the one end portion of the inner circumferential surface has a curved shape portion extending away from the one end portion of the outer circumferential surface to form a projection that projects toward the other end of the coil, such that an area of a cross-section of the coil side passage orthogonal to a traveling direction of the cooling medium increases along the traveling direction of the cooling medium from a beginning of the curved shape portion, which is closest to the inlet, to a position within the curved shape portion and decreases along the traveling direction of the cooling medium from the position within the curved shape portion to an end of the curved shape portion which is further away from the inlet than the beginning of the curved shape portion.