To reduce the whole man hours necessary to check the defect of a work by detecting, with a few man hours, the occurrence of sparks and overheating that can be generated when the work is electrically heated.

A total of four optical sensors 10-13 are installed around contact parts between a work W and respective electrodes 3a-3d, and they detect luminance from the vicinity of the contact parts. The luminance detected by each sensor 10-13 is input into an electric conduction spark detection device 6. The electric conduction spark detection device 6 obtains luminance (background luminance) before electric conduction heating start on the basis of detection signals of the respective optical sensors 10-13 before electric conduction heating by electric conduction from a power supply for heating 2 to the work W is started. The electric conduction spark detection device compares luminance after the electric conduction heating start (luminance at the time of heating) to the background luminance per optical sensor 10-13, and determines that a spark has occurred if the situation is an excess luminance state in which the difference between them is larger than a luminance increment threshold value.