A light source device configured as a master oscillator power amplifier includes a mode locked laser unit having an external resonator and a semiconductor optical amplifier that amplifies and modulates laser light emitted from the mode locked laser unit. The width in a lateral direction of a waveguide on an incident side of the semiconductor optical amplifier is set so that a horizontal lateral mode of the waveguide on the incident side of the semiconductor optical amplifier becomes multiple modes, and a magnification conversion unit that converts a magnification of incident light from the mode locked laser unit to the semiconductor optical amplifier is disposed so that a basic mode is selectively excited in optical coupling on the incident side of the semiconductor optical amplifier.