To provide a heat treating apparatus capable of controlling the temperature of a substrate to be treated with high accuracy, without picking-up heat except the heat generated from the substrate to be treated as noise.

A sensor insertion hole 21 is bored from the side surface of a heat treatment board 2, in parallel with the surface of the board 2 and directed to the central part of the board 2. A temperature sensor 30 is inserted in the hole 21 and fixed. The tip of the hole 21 is continuously connected with a heat ray inlet hole 22, which is bored from the upper surface of the heat treatment board 2 perpendicularly to the surface of the board 2, and a reflecting part 23 which is inclined at almost 45 degrees with respect to the upper surface of the heat treatment board 2 is formed in a part on which the sensor insertion hole 21 and the heat-ray introducing hole 22 are coupled. Heat ray generated from a wafer W is introduced in the heat ray inlet hole 22, reflected by the reflecting part 23, introduced into the temperature sensor 30 in the sensor inserting hole 21 and then detected.