A photo SI thyristor driving circuit which has a good switching characteristic suitable to be used for a photo relay, and a protection circuit therefor for protecting a photo SI thyristor element from being broken down due to a thermal runaway. The photo SI thyristor driving circuit comprises a photo SI thyristor, a MOS transistor provided in a gate circuit of the photo SI thyristor to draw charges accumulated at the gate of the photo SI thyristor when the photo SI thyristor is turned off, and a plurality of photo electromotive force elements provided in a gate circuit of the MOS transistor to generate a photo current for biasing the MOS transistor. The protection circuit includes a snubber circuit connected between the anode and the cathode of the photo SI thyristor and a light emitting element for irradiating light to the photo electromotive elements in response to an output from the snubber circuit.