In a refractive index coupling distributed semiconductor laser having a Λ/2-phase-shift distributed feedback structure with a diffraction grating having a refractive index coupling property on an active layer, when viewed in a light distributed feedback direction, a value of (duty of a high refractive index portion)/(duty of a low refractive index portion) of a diffraction grating in a rear end face region is larger than that of a diffraction grating in a front end face region. In this manner, a coupling coefficient κ2 in a front end face region of a conventional semiconductor laser is smaller than a coupling coefficient κ1 in a rear end face region and is larger than 100 cm−1.