A method whereby a plurality of inverters for converting DC power outputted from a DC power supply, such as solar cells or fuel cells, to AC power are operated efficiently without being biased to particular inverters. The number of inverters to be run is determined in correspondence with at least one output value of the DC output or AC output, and the determined number of inverters are selected and made to run from among the plurality of inverters on the basis of a predetermined rule. In addition, a parallel-connected system is disclosed for enabling efficient and appropriate parallel-in operation by the use of inverters, with one of the inverters controlling the remaining inverters and effecting system interconnection protection.