PURPOSE: To prevent detection delay simply, by monitoring a voltage across the anode terminal and the gate terminal of each GTO thyristor constituting a switching element in a power converter device, and judging ON and OFF of the GTO thyristor.

CONSTITUTION: An anode terminal A and a gate terminal B of each GTO thyristor G constituting a switching element in a power converter device are connected to a state monitoring circuit DT. The state monitoring circuit DT is constituted by protecting resistors R1 and R2 and photocoupler PC. When an anode-gate voltage VAG of the GTO thyristor G becomes a specified value or more, a light emitting element P1 of the photocoupler PC emits light, and a light receiving element P2 is turned ON. An output signal S becomes a 0. Thus the ON state of the GTO thyristor G is judged. Conversely, when the voltage VAG becomes lower than the specified value, the output S of the photocoupler PC becomes a 1. Thus delay in detecting time is prevented readily, and the device can be made compact.