A semiconductor laser has first and second diffractive grating regions. The first diffractive grating region has segments, has a gain, and has first discrete peaks of a reflection spectrum. The second diffractive grating region has segments combined to each other, and has second discrete peaks of a reflection spectrum. Each segment has a diffractive grating and a space region. Pitches of the diffractive grating are substantially equal to each other. A wavelength interval of the second discrete peaks is different from that of the first discrete peaks. A part of a given peak of the first discrete peaks is overlapped with that of the second discrete peaks when a relationship between the given peaks of the first discrete peaks and the second discrete peaks changes. A first segment located in the first diffractive grating region or the second diffractive grating region has an optical length shorter or longer than the other segments of the first diffractive grating region and the second diffractive grating region by odd multiple of half of the pitch of the diffractive grating of the first diffractive grating region.