The present traverse hardening device performs traverse hardening on a shaft-like body which includes a main body part, a first small-diameter part provided at an end portion of the main body part on one side in an axial direction and having an outer diameter smaller than that of the main body part, and a second small-diameter part provided at an end portion of the first small-diameter part on one side in the axial direction and having an outer diameter smaller than that of the first small-diameter part. The traverse hardening device includes: a primary coil member which is formed in an annular shape, through which a high-frequency current is caused to flow, and which is externally disposed around the main body part; a secondary coil member formed in an annular shape, externally disposed around the first small-diameter part of the shaft-like body, and being able to be disposed in the primary coil member; and a tertiary coil member formed in an annular shape, externally disposed around the second small-diameter part of the shaft-like body, and being able to be disposed in the secondary coil member.