An optical device is disclosed in which inner space is kept clean so as to reduce the adhesion and adsorption of pollutants to the surface of the optical device, thereby preventing the degradation of the optical device and accurately maintaining the alignment of the coupling between the laser light from a semiconductor laser and the incident end surface of an optical fiber. The optical device includes a lens system unit condensing the laser light emitted from the semiconductor laser and guiding the laser light onto an incident end of the optical fiber; a variable volume mechanism unit defining an hermetically enclosed space; and an adsorbent unit disposed in the hermetically enclosed space, in which either the incident end of the optical fiber or the lens system unit or both are in contact with the hermetically enclosed space containing the adsorbent unit.