The optical semiconductor device comprises a lower clad layer 12, optical waveguide layers 14, 16, 18 including an active layer for recombining the carriers. The upper clad layer 20 is mesa stripe configuration having a first mesa portion contacting the contact layer 22 and having a first width, and a second mesa portion having a second width larger than the first width. The first width, the second width and the thickness of the second mesa portion are set not to oscillate in the higher-order transverse mode.