PURPOSE: To prevent damage of a gap sensor and to improve quenching workability, by providing control means for bringing the gap sensor to contact with or leave from a surface to be treated of a work, based on detected signal of the surface part to be treated of the work.

CONSTITUTION: End part detecting sensors 19, 22 are provided in addition to the gap sensors 4, 5 to a matching unit 2, to detect end part of the surface 1a to be treated of the work 1. A control part 16 operates air cylinders 26, 27 due to detected signal of the sensors 19, 22, and the sensors 4, 5 are brought to contact with or leave from the surface 1a of the work 1. In this way, high frequency quenching can be carried out while correcting and maintaining the gap between the surface 1a and a work coil 3 to a suitable value.