An external cavity semiconductor laser is provided with a first input-output section 2a and a second input-output section 2b that are disposed on a common end surface 2 via a center axis 3. The external cavity semiconductor laser includes: a semiconductor laser device 1 capable of emitting a first laser light 4 and a second laser light 5 from the input-output sections 2a and 2b in two directions; a first reflecting unit for reflecting the laser light 4 emitted from the first input-output section 2a and returning the reflected laser light 4 to the first input-output section 2a; a second reflecting unit 14b for reflecting the laser light 5 emitted from the second input-output section 2b and returning the reflected laser light 5 to the second input-output section 2b; a third reflecting unit 8 provided to a second end surface 7 disposed opposite a first end surface 2 having input-output sections 2a and 2b of the semiconductor laser device 1 for reflecting the laser light having returned to one of the input-output sections so as to be emitted from the other one of the input-output sections; and a halfmirror 13 outputting a part of the laser light outputted therefrom.
