a gate driver is provided for controlling a gate voltage of each of a plurality of mos control semiconductor devices , such as igbts or metal oxide mos transistors , of a semiconductor power converter in which said mos control semiconductors are connected in series with each other , the gate driver includes a power supply line having a higher potential than a gate potential on each of said mos control semiconductor devices when in steady on state , and an arrangement for supplying a current from the power source line to the gate of each of said mos control semiconductors to increase the gate voltage of the mos control semiconductor devices when a potential difference between said power supply line and an emitter of each of said mos control semiconductors is constant and when a collector voltage of the mos control semiconductor device exceeds a predetermined value under on state of the mos control semiconductor device .