Provided is a converter controller capable of preventing destruction of an element such as an auxiliary switch by preventing operation interference between auxiliary circuits of respective phases in a multiphase soft switching converter. A duty threshold input unit receives, as an input, an obtained acceptable duty deviation value. A duty deviation computation unit judges whether or not the duty deviation between the phases does not exceed an acceptable duty deviation value. When the duty deviation between the phases exceeds the acceptable duty deviation value, the duty deviation computation unit corrects an adjusted U-phase duty ratio, adjusted V-phase duty ratio and adjusted W-phase duty ratio under the PID control rule, and outputs the resultant duty ratios to an FC converter control circuit. On the other hand, when the duty deviation between the phases does not exceed the acceptable duty deviation value, the duty deviation computation unit does not correct the adjusted U-phase duty ratio, adjusted V-phase duty ratio and adjusted W-phase duty ratio and outputs them to the FC converter control circuit.