PURPOSE: To improve the accuracy in the measurement of a m.p. and to facilitate the operation for the measurement by placing a sample atop a projecting part and measuring the surface temp. thereof during heating and measuring quickly the temp. in the place where the sample is placed when the sample melts and moves to the lower part.

CONSTITUTION: A heater 6 is formed by bending a thin sheet of tungsten at plural points and providing a recess 7 in the central part. The central part in the recess 7 is projected to a trapezoidal shape to form a sample setting part 8, the circumference thereof is formed as a low part 9 via a step 10. Such heater 6 is set in a chamber filled with a reducing atmosphere (He+4.0% H2) and a uranium dioxide pellet is placed in the part 8. When the heater 6 is electrically heated to elevate the temp. of the pellet, the pellet melts and rounds until it slips down the step 10 and moves from the part 8. The temp. of the part 8 is measured in this state by using an automatic two color type non-contact thermometer from right above the position where the sample is set. In such a way, the temp. at which the sample pellet is heated is exactly measured immediately after the molten pellet has moved.