To provide a heat treatment device that can prevent the reduction of sealing performance in a sealing part.

The heat treatment device is provided with a chamber body 6 wherein a space for heat treatment of a substrate is formed by a chamber side 63 having a nearly cylindrical inner wall, a chamber bottom covering the lower part of the chamber side 63, and an opening on its upper side. It is also provided with a light irradiation part to emit a light to the substrate within the chamber body 6 through the opening and a light transmitting plate 61 that blocks the opening of the chamber body 6 and transmits a light from the light irradiation part. A circular groove 631 is formed on the upper end surface of the chamber side 63 and a circular metallic gasket 51 is provided within the circular groove 631. The metallic gasket 51 is pinched between the light transmitting plate 61 and the chamber side 63 and is elastically deformed, thereby sealing a space therebetween. Thus, sealing performance between the chamber body 6 and the light transmitting plate 61 in the heat treatment device can be prevented from being reduced.