PURPOSE: To suppress the increase of a threshold value in order to shorten the life in the saturable absorption area with respect to an optical bistable semiconductor laser which is one of the optical signal processing devices in the optical communication and optical network.

CONSTITUTION: A bistable semiconductor laser is provided with gain areas A and B where the carriers are implanted, and a saturable absorption area C which is located between the gain areas A and B. The structure of the laser includes an arrangement that the active layer 4 in the saturable absorption area C is a high-density dopant region 4a having a density higher than the dopant density of the active layer 4 in the gain regions A and B.