Tunable laser source apparatus having wideband oscillation wavelength continuous sweep function

A tunable laser source apparatus includes an external cavity. The external cavity includes a semiconductor laser (11) and a diffraction section (2). At least one laser output end facet (11b) of the semiconductor laser (11) is covered with an AR coating. The diffraction section (2) includes a first reflector (22) and a diffraction grating (21), and has wavelength selectivity with which light emerging from the end facet (11b) of the semiconductor laser (11) which is covered with the AR coating is received, and light having a predetermined wavelength is selected and reflected toward the semiconductor laser (11). The semiconductor laser (11) further includes an angle detection section (4) and a control section (5). The angle detection section (4) detects the angle defined by the optical path of the light emerging from the semiconductor laser (11) and the optical path of the diffracted light reflected by the diffraction section. The control section (5) controls at least the cavity length of the external cavity or the selected wavelength of the diffraction section (2) to set the angle detected by the angle detection section (4) to zero.