Abstract: A hardening apparatus comprises a tracing gauge that is a member having the same axis shape as the axis shape of a bent pipe and being held in an attitude identical to the bent pipe, and a tracing device that is a member which can move in the longitudinal direction of the bent pipe and the tracing gauge and can move in the direction perpendicular to the longitudinal direction. A guide part engaged with the tracing gauge in a manner slidable along the tracing gauge is held at one end of the tracing device, and a hardening device is held at the other end. A transportation device moves the hardening device in the longitudinal direction by moving the tracing device.