As illustrated in FIG. 3, the main body 1 includes a power output terminal 41 electrically connected to the main body power supply portion 18. Power (DC 24V) generated in the main body power supply portion 18 is applied to the power output terminal 41. Each of the expansion devices 2 has a power input terminal 42 electrically connected to the power conversion circuit 23. Power (DC 24V) inputted to the power input terminal 42 is applied to the power conversion circuit 23. In addition, the expansion device 201 has a power output terminal 44 electrically connected to the expansion power supply portion 19. Power (DC 24V) generated in the expansion power supply portion 19 is applied to the power output terminal 44. Furthermore, the expansion device 201 has a power supply terminal 43 electrically connected to the power input terminal 42. Power (DC 24V) inputted to the power input terminal 42 is applied to the power supply terminal 43.