To ensure high quality heat treatment of a large work.
A heat treatment chamber 2 for a work W, a wind tunnel 4 formed through a heat storage plate 3, a temperature rising heater 5 being turned on upon temperature rise, and a normal hot air circulation system are arranged in a body 1. The heat treatment chamber 2 is provided, at the inlet and outlet thereof, with an air flow regulation plate 21 and a porous plate 22 while the wind tunnel 4 is provided with an wind tunnel inlet 41 and a porous plate 42 having smaller opening area than the porous plate 22. The heat storage plate 3 has a large number of vents 31. In case of a large work, the flow regulation plate is closed and hot air from the wind tunnel strikes against the lower surface of the work through the vents before being discharged from the porous plate 22. Since no wind flows on the upper surface of the work, the work can be fired with high quality and heat exchanging performance 15 enhanced through turbulence on the lower surface. Temperature of the work can be raised with good temperature distribution by means of the heat storage plate and hot air supply from a large number of vents. Temperature rising rate is increased by means of the temperature rising heater 5 and the efficiency of heat treatment is enhanced.