A signal output circuit includes a signal conversion element, a signal output transistor, a first resistor element, a second resistor element, a diode, a malfunction-prevention transistor, and a capacitance-compensating capacitor. The signal output transistor establishes electrical conduction between two conductive terminals when an on signal is supplied to a conduction control terminal via the signal conversion element. The diode is connected in parallel to the second resistor element in a direction in which current flows between a signal output terminal of the signal conversion element and the conduction control terminal of the signal output transistor when an on signal is outputted to the signal output terminal. The malfunction-prevention transistor establishes electrical conduction between two conductive terminals when a current flows through the second resistor element in a direction opposite the direction of the diode. The capacitance-compensating capacitor is connected in parallel to the second resistor element.