A light source device comprising: a plurality of external-cavity laser modules configured to emit a plurality of laser beams of different peak wavelengths, the plurality of external-cavity laser modules comprising at least one first laser module and a plurality of second laser modules; and a beam combiner configured to coaxially combine the plurality of laser beams to generate a wavelength-combined beam, wherein: each of the plurality of external-cavity laser modules comprises: a collimation laser light source having a Littrow configuration, and a diffraction grating configured to selectively reflect and transmit light of a predetermined wavelength, the plurality of external-cavity laser modules are arranged so that the plurality of laser beams are incident on a same region of the beam combiner at different angles, a first distance (La) between the at least one first laser module and the beam combiner is different from a second distance (Lb) between the plurality of second laser modules and the beam combiner, the at least one first laser module is arranged such that one end of the at least one first laser module is located on a circumference of a circle whose radius is equal to the first distance (La), and the plurality of second laser modules are arranged such that one end of each of the second laser modules is located on a circumference of a circle whose radius is equal to the second distance (Lb), wherein: the plurality of external cavity laser modules are arranged so that each laser beam emitted from the at least one first laser module passes through a space between corresponding two adjacent second laser modules of the plurality of second laser modules; and a smallest interval between two adjacent second laser modules of the plurality of second laser modules is narrower than a width of a corresponding one of the at least one first laser module at an emitting end of the corresponding one of the at least one first laser module. a plurality of external-cavity laser modules configured to emit a plurality of laser beams of different peak wavelengths, the plurality of external-cavity laser modules comprising at least one first laser module and a plurality of second laser modules; and a beam combiner configured to coaxially combine the plurality of laser beams to generate a wavelength-combined beam, wherein: each of the plurality of external-cavity laser modules comprises: a collimation laser light source having a Littrow configuration, and a diffraction grating configured to selectively reflect and transmit light of a predetermined wavelength, a collimation laser light source having a Littrow configuration, and a diffraction grating configured to selectively reflect and transmit light of a predetermined wavelength, the plurality of external-cavity laser modules are arranged so that the plurality of laser beams are incident on a same region of the beam combiner at different angles, a first distance (La) between the at least one first laser module and the beam combiner is different from a second distance (Lb) between the plurality of second laser modules and the beam combiner, the at least one first laser module is arranged such that one end of the at least one first laser module is located on a circumference of a circle whose radius is equal to the first distance (La), and the plurality of second laser modules are arranged such that one end of each of the second laser modules is located on a circumference of a circle whose radius is equal to the second distance (Lb), wherein: the plurality of external cavity laser modules are arranged so that each laser beam emitted from the at least one first laser module passes through a space between corresponding two adjacent second laser modules of the plurality of second laser modules; and a smallest interval between two adjacent second laser modules of the plurality of second laser modules is narrower than a width of a corresponding one of the at least one first laser module at an emitting end of the corresponding one of the at least one first laser module.