The semiconductor device of the present invention comprises: a transistor in which voltage is applied between a drain and a source by a high-voltage power source; a drive device for generating gate voltage and source voltage for the transistor from voltage of a low-voltage power source having lower voltage that the high-voltage power source; and a voltage-dividing circuit connected to the low-voltage power source, the semiconductor device being characterized in that the output voltage of the voltage-dividing circuit is applied to the source when the source voltage is less than a fixed value.