An optical semiconductor device includes a waveguide having one or more first segments having a region that includes a diffractive grating and another region combined to the region, one or more second segments having a region that includes a diffractive grating and another region combined to the region and a plurality of third segments having a region the includes a diffractive grating and another region combined to the region, a length of the second segment being different from that of the first segment, a length of the third segment being shown as L3=L1+(L2−L1)×K1 in which 0.3≦K1≦0.7, L1 is a length of the first segment, L2 is a length of the second segment and L3 is a length of the third segment; and a refractive index control portion controlling refractive index of the first segment through the third segments.