The BaPb0.7Bi0.3O3sputtered film possesses tunnel Josephson junctions at boundary layers [boundary Josephson junction (BJJ)] normal to the film plane in a homogeneous junction array. The film has high efficiency for optical irradiation of the junctions because of the high optical transparency. The letter presents the optical effect on the current‐voltage characteristics for this Josephson junction array locked to a microwave field. The microwave‐induced hysteresis loop caused by voltage locking among junctions in a microwave field is highly sensitive to optical illumination with as low an incident power as a few nanowatts. This probably can be exploited in a future, highly sensitive photodetector.