PURPOSE: To improve working efficiency and quality by providing photodetecting parts near a high frequency coil which heats a pipe or the like, measuring the surface temp. distributions of the pipe or the like with an optical temp. measuring device, and so controlling the relative positions of the high frequency coil and the pipe or the like that they are changed by coil driving devices in correspondence to the temp. distributions thus measured.

CONSTITUTION: Photodetecting parts 21W24 of an optical temp. measuring device 10 which measures the surface temp. of a pipe 1 are provided near a high frequency coil 4. The beams of light received in the parts 21W24 are entered successively one by one into the photoelectric transducer 11 of the device 10 via an optical switching device 8. The device 10 emits the output voltages of the temps. corresponding to the entered light energies time-dividedly from an output terminal 9. These output voltages are extracted by sample holding circuits 31W34, are compared in comparators 13, 14, and control coil driving devices 5, 6, thereby keeping away the coil 4 from the high temp. part of the pipe 1 and bringing the same approached to a lower temp. part.