A semiconductor optical device includes a waveguide layer and a reflecting multi-layer film. The waveguide layer includes two cladding layers and an active layer sandwiched between the two cladding layers. The reflecting multi-layer film including multiple layers is on at least one of a pair of opposing end faces of the waveguide layer. A summation n_id_i of products n_id_i of refractive indexes n_i and thicknesses d_i of the layers denoted i in the reflecting multi-layer film, and a wavelength ₀ of light guided through the waveguide layer satisfies a relationship, n_id_i0/4. A first wavelength bandwidth is wider than a second wavelength bandwidth . is a wavelength range including the wavelength ₀ in which a reflectance R of the reflecting multi-layer film is not higher than +2.0% from reflectance R at the wavelength ₀. is a wavelength range including the wavelength ₀ in which a reflectance R of a hypothetical layer is not higher than +2.0% from a hypothetical reflectance R at the wavelength ₀ of a hypothetical layer having a thickness of 5₀/(4n_f), a refractive index n_f, on the at least one of opposing end faces, and satisfying a relationship, R=((n_c-n_f²)/(n_c+n_f²))², where n_c denotes an effective refractive index of the waveguide layer.