NOVELTY - The inverter (2) has a control circuit (8) for controlling opening and closing of a contactor (7). The control circuit detects whether a value of a direct current (DC) voltage output from a DC voltage conversion circuit (4) is equal to or greater than a desired threshold value. The control circuit comprises a switch abnormality decision circuit (8a) for deciding whether contactor condition is abnormal condition i.e. contact cohesion, after the contactor is opened by the control circuit when the value of the DC voltage is equal to or greater than the threshold value.    USE - Grid-connected inverter for converting DC electric power generated by a solar cell into alternating current (AC) electric power to be supplied to a load through a commercial power system in a grid-connection system i.e. solar cell power generation system. Can also be used for a fuel cell power generation system.    ADVANTAGE - The inverter decides presence or absence of abnormality of the contactor by using circuit voltage used for operation control such that the need for additionally providing a specialized unit for detecting the presence or absence of abnormality is avoided. The presence or absence of the abnormality in the circuit is decided based on voltage across a charged capacitor rather than minute current flowing in a filter circuit to avoid miss-detection caused by a noise, thus achieving highly reliable grid-connected inverter without increasing size and cost of the inverter. The abnormality can be found before starting an operation of the inverter to enable protection of secondary failure and enable safer disconnection from the system. The inverter uses a resistor for discharging electric charges stored in the capacitor at maintenance work of the inverter, thus ensuring safety of workers.    DESCRIPTION OF DRAWING(S) - The drawing shows a circuit block diagram of a solar cell power generation system.    Grid-connected inverter (2)    DC voltage conversion circuit (4)    Contactor (7)    Control circuit (8)    Switch abnormality decision circuit (8a)