Abstract: The present invention relates to a single mode light source device having an external cavity. The single mode light source device according to the present invention includes a laser diode, an integrated lens, and an optical fiber. The optical fiber has an incidence surface cleaved at a predetermined angle, such that a portion of light emitted from the laser diode is reflected to return to the laser diode. An internal cavity embedded in the laser diode is combined with an external cavity, thereby selecting only an optical signal of a single mode (wavelength) and outputting the selected optical signal. The external cavity is formed between a light emitting surface of the laser diode using multi mode oscillation and the incidence surface of the optical fiber. According to the present invention such constructed, the single mode light source can be embodied easily at a low price and also can be used as a variable wavelength light source.