A parallel power system is provided which can flexibly change output currents and the number of channels in short time and at low cost. A back board 7 of the parallel power system has connectors ECC and DCC for plug-in mounting power source circuits 1 to 6 and an electronic circuit 8. Terminals of the connectors DCC and EDC are connected by wiring lines HP. The power source circuits 1 to 3 constitute a first voltage channel by a power source line DH1 and a parallel operation control line PCH1 of the electronic circuit 8, whereas the power source circuits 4 to 6 constitute a second voltage channel by a power source line DH2 and a parallel operation control line PCH2. When a specification of the electronic circuit is to be changed, only the power source lines DH1 and DH2 and parallel operation control lines PCH1 and PCH2 are changed without changing the back board 7 and the like.