PURPOSE: To smoothly carry out the productive control for reflow soldering by measuring the maximum surface temperature of a material to be heated, comparing the value with the average value of output signals of a radiation thermometer to adjust an output and using the radiation thermometer to monitor the surface temperature of an actual product.

CONSTITUTION: The material 9 to be heated is carried into a reflow furnace 1. The maximum surface temperature is measured by a thermocouple in the material 9 to be heated in this actual heating area 4 to obtain the average value of the temperature output signal of the radiation thermometer 10. These measured values are compared with the average value and the output of the radiation thermometer 10 is controlled so that the average value of the temperature output signals is equivalent to the measured value of the maximum surface temperature. Thereafter, the surface temperature of the material (product) 9 to be heated carried into this actual heating area 4 is monitored. In this way, the temperature width to be monitored can be set suitably.