In addition, the fluctuation generating portion 31 need not to fluctuate the output power of the main body power supply portion 18, and may fluctuate the output power of the expansion power supply portion 19. In this case, in the load (expansion device 2), when a fluctuation that correlates with a fluctuation of the output power of the power supply portion (expansion power supply portion 19) appears in the supply power, the connection identifying portion 32 identifies that the connection destination of the load is the expansion power supply portion 19. In addition, the fluctuation generating portion 31 may respectively fluctuate, at different timings, the output power of the main body power supply portion 18 and the output power of the expansion power supply portion 19, or may generate fluctuation with different amplitudes or time lengths. Three or more power supply portions are acceptable.

In addition, the operation for identifying the connection relationship between the power supply portion and the load may be performed not only immediately after the user completes the set-up including the connection work and turns on the power, but also at other timings. For example, a predetermined operation by the user or energization to an expansion load may be set as a trigger to start the operation for identifying the connection relationship between the power supply portion and the load.