To generate no error in a burning-preventive operation of a coupler by performing power supply in compliance with a decision of an overheating decision means.

A coupler temperature estimation means 8 for calculating a present temperature rise from an AD-converted effective current and a thermal constant of a coupler 1 for the preliminary set power transmission and a set value 9 of a temperature rise of the coupler being preliminary set inside a memory are provided. Further, a comparison means 10 comparing with a temperature rise set up in advance by the coupler temperature estimation means 8 and a feeding decision means 11 for deciding whether or not feeding to the coupler 1 for power transmission when the present temperature rise of the coupler 1 for power transmission becomes larger than the set-up value 9 according to the comparison means 10 are provided. Then, on the fasis of the feeding decision means 11, abnormality of the coupler 1 for power transmission is given off by an alarm 12. Therefore, stable burning-preventive operation can be performed.