PURPOSE: To efficiently recover the hardening salt from a washing tank by thickening the hardening salt-contg. washing water in a washing tank by a thickening means and returning the thickened hardening salt-contg. washing water to a hardening tank.

CONSTITUTION: The metallic member quenched in the hardening tank 11 contg. the fused hardening salt as a coolant is transferred by a conveyor 13 to the washing tank 12 where the member is washed by the washing water. The washed member is taken out of the tank to the outside. The hardening salt-contg. washing water is taken out of the washing tank 12 and is preheated by a heater 15 when the hardening salt in the washing water exceeds a specified ratio. This water is further heated in an evaporation vessel 16 to evaporate the moisture and is made into the washing water contg. the thickened hardening salt. This water is returned by a pump P to the hardening tank 11. On the other hand, the moisture evaporated in the evaporating chamber 16 is condensed by condensing plates 17 provided in plural stages to the condensate. The gathered condensate is returned by the pump P to the washing tank 12. The hardening salt carried out of the hardening tank 11 to the washing tank 12 is efficiently recovered in this way.