A side surface unit of a heat treatment space S is formed by a shutter member 250 including an outer shutter 260 and an inner shutter 270. Supply air A is supplied as a horizontal laminar flow toward a wafer W from a lower end side of the shutter member 250, that is, from a gap d1 located on the level with the wafer W placed on a heat plate 211 of a mounting table 210. Supply air B is supplied into the heat treatment space S from an upper end side of the shutter member 250, that is, from a gap d2 positioned higher than the wafer W. A ratio between a flow rate of the supply air A and a flow rate of the supply air B is 4:1.