An external cavity semiconductor laser including a first and second input-output section disposed on a common end surface via a center axis, and a semiconductor laser device emitting a first and second laser light from the first and second input-output sections in two directions. The laser device includes a first reflecting unit for reflecting the first laser light emitted from the first input-output section and returning the reflected first laser light to the first input-output section, a second reflecting unit for reflecting the second laser light emitted from the second input-output section and returning the reflected second laser light to the second input-output section, a third reflecting unit provided to a second end surface disposed opposite a first end surface having first and second input-output sections for reflecting laser light returned to one of the input-output sections so as to be emitted from the other one of the input-output sections.