A solid state relay includes a MOS FET receiving a photovoltaic output generated across a photovoltaic diode array responsive to a light signal from a light-emitting element, and a normally ON driving transistor connected to the MOS FET, the driving transistor being connected at control electrode to a connection point between the photovoltaic diode array and an impedance element to be biased by a voltage generated across the impedence element during generation of the photovoltaic output across the photovoltaic diode array to have a high impedance state, whereby the relay can be prevented from providing at output terminals any intermediate state between ON and OFF states even when an input current to the relay is in lower range, and a high speed relay operation is realized.