PURPOSE: To detect continuous lighting of a light emitting element being a component of a light emitting element drive circuit due to a defect, a fault or a wiring defect of a component of the light emitting element drive circuit built in the electrooptic conversion circuit.

CONSTITUTION: Since the electrooptic conversion circuit is provided with a light receiving element 12b detecting the lighting state of the light emitting element 12a integrated with a light emitting element drive circuit 12 and being a component of the light emitting element drive circuit and with a time monitor circuit 9A monitoring the lighting time of the light emitting element based on the result of detection of the light receiving element and monitors directly the light emission of the light emitting means, when the light emitting element is faulty or deteriorated, the state is easily detected.