PURPOSE: To enable the temperature distribution in a surface to be readily measured, by providing the first and second plate-shaped bodies having the first and second grooves of shapes different from each other and a thermocouple which is installed in a heat-treatment apparatus and whose position is controlled through said grooves.

CONSTITUTION: A pair of disks 10 facing to each other are connected with supporting rods 11, while a pair of disks 4 facing to each other are connected with supporting rods 12. A thermocouple 3 is inserted into rectilinear grooves 6 in the disks 4 and S-shaped grooves 7 in the disks 10, and the whole is inserted into a core tube 2 in an electric oven 1. In other words, the thermocouple 3 can freely move in the grooves 6 and 7 in said constitution. Accordingly, if the in-surface axial direction designating disks 4 and the position setting disks 10 are placed so that the thermocouple 3 is located in the central part in the surface, the thermocouple 3 moves in the core tube 2 according to the S-shaped grooves 7, so that the temperatures in the horizontal direction in the surface can be readily measured.