To provide an optical semiconductor device which makes a long- distance transmission characteristic uniform and which enhances the yield of the transmission characteristic after a long-distance transmission, by suppressing the dispersion of an extinction ratio generated when different-wavelength EA modulator integrated DFB lasers which are operated at mutually different wavelengths are formed collectively on one semiconductor substrate.

An InGaAsP guide layer 104, an MQW active layer 105 and a p-InP clad layer 106 are selectively grown on an n-InP substrate 101 by a selective MOVPE method by using a pair of SiO2 stripe masks 103, in such a way that the width of the stripe masks 103 is made wide according to an oscillation wavelength. The width of the mask 103 in a DFB laser region and that of the mask 103 in an EA modulator region are adjusted so as to be a detuning amount by which the extinction ratio of each semiconductor laser and that of a pair of electric field absorption optical modulators become nearly equal.