A level determination circuit 60 determines whether a voltage level Vref of an output variable signal is a previously set reference level Vref−m or higher. According to this determination result, a constant voltage output circuit 70 and a constant current output circuit 80 selectively operate. The constant voltage output circuit 70 outputs a transfer voltage Vout of a level corresponding to the voltage level Vref of the output variable signal. The constant current output circuit 80 outputs a constant current Iout of a level corresponding to the voltage level Vref of the output variable signal.