In an external mirror type semiconductor laser composed of a semiconductor laser having a structure wherein a reflection factor is reduced on one end plane thereof, and a lens as well as a grating type reflector disposed on the side of the end plane, respectively, the lens is shifted in the direction orthogonal to a light axis, and an incident angle of the light outputted from the end plane to the grating type reflector is changed, whereby an external mirror type wavelength tunable laser having a simple structure and tuning a radiated wavelength can be realized.