A light source device includes a microwave generating section for generating a microwave, a reflector connected to the microwave generating section, and a microwave electrodeless lamp disposed inside a cavity, the cavity including the reflector and a metallic chamber covering an opening section of the reflector. The chamber includes a tube member having an opening section for communicating the inside and the outside of the cavity to lead a light flux out of the chamber, and a bore diameter, D of the opening section, a length L of the tube member, a wavelength λ of the microwave, and a wavelength Λ of the light flux satisfy Λ