single control switch for activating a power source for supplying a power to the driving circuit or in particular to the control system in the present instance an interruption of power from a battery b as the control power source to the circuit with the control switch sw1 turned off causes the gate voltage of the lower side power mosfet 39 s tr19 to tr21 to disappear first and these transistors are all turned off next the transistors tr13 to tr15 for a level conversion between the side of the floating power sources 12 1 to 12 3 for driving the upper side power mosfet 39 s tr1 to tr18 and the distributor circuit 11 are turned off due to the loss of their base current three pairs of totem pole transistors tr1 and tr2 tr3 and tr4 and tr5 and tr6 for driving the gates of the upper side power mosfet 39 s tr16 to tr18 are connected at their respectively bases with a corresponding one of the floating power sources 12 1 to 12 3 the capacitors c1 to c3 in the three floating power sources 121 to 123 are maintained in a state in which the source voltage is kept applied without discharging their accumulated charge even when the power supply from the battery b is interrupted as a result the respective totem pole transistors are biased at their base through resistors r16 r22 and r28 or in other words the npn transistors tr1 tr3 and tr5 in the transistor bridges 13 1 to 13 3 are turned on while the pnp transistors tr2 tr4 and tr6 are turned off accompanying the above the charge in.