PURPOSE: To enable sure detection of a state of overload by applying a filter processing to a detected value of an invertor output current only when a load current lowers.

CONSTITUTION: A detected value (Id) (solid line) of a load current converted by an A/D converter 8 is compared with an output value (If) (dotted line) of a filter. When the If to be compared with the Id(m) is used as If(m-1) obtained by applying a filter processing to Id(m-1) sampled one time befor, in comparison of it with the ID, for instance, the Id(m) sampled this time is larger than the If(m-1) sampled one time before in a state of the load current being unvaried or rising. In a state of the current being lowering, the Id(m) is smaller than the If(m-1) sampled one time before. In other words, fluctuation in the load current can be detected from comparison of the Id with the If. At the time when the load current rises or is unvaried, the detected value of the load current after the A/D conversion to be compared with a set reference level value of an overload current is set at the If (dotted line). At the time when the load current lowers, the filter processing is conducted. Accordingly, a state of overload can be detected reliably.