A tunable external resonator laser comprising: a laser chip which emits light, said laser chip having a rear end surface mirror; a lens which collimates the light emitted from said laser chip; a diffraction grating which diffracts the light collimated by said lens; a support body onto which said diffraction grating is fixed; and a movable mirror which reflects the light diffracted by said diffraction grating; wherein said diffraction grating is arranged at a prescribed distance (D G) from a Littman-type tunable external resonator laser arrangement in which said movable mirror is configured to rotate around a pivot which is the intersection point of the surface of said movable mirror and the surface of the said diffraction grating, said prescribed distance (D G) being along a direction parallel to the normal direction to the grating surface in said Littman-type arrangement, so that mode-hop is suppressed, characterized in that the rear end surface mirror of said laser chip is moved by a predetermined offset Δ in a direction parallel to the emitted light from said laser chip, wherein the prescribed distance is equal to an offset D G of the pivot and the surface of said diffraction grating, said offset D G is determined by the following formula: D G = d 2 n 1 λ dλ 2 | λ 0 l + d 2 n 2 λ dλ 2 | λ 0 t 1 − λ 0 d − sin α 2 3 d 2 wherein: l 0 is desired center wavelength, n 1 ( λ) is effective refractive index of waveguide mode of laser chip, n 2 (λ) is refractive index of collimation lens, d 2 n 1 (λ) / dλ 2 or d 2 n 2 (λ) / dλ 2 is respectively quadratic differential coefficient for each wavelength of refractive index, l is the chip length of laser chip, t is effective thickness of collimation lens, d is interval of groove of diffraction grating, α is incident angle between collimated light and diffraction grating which collimating light is incident. a laser chip which emits light, said laser chip having a rear end surface mirror; a lens which collimates the light emitted from said laser chip; a diffraction grating which diffracts the light collimated by said lens; a support body onto which said diffraction grating is fixed; and a movable mirror which reflects the light diffracted by said diffraction grating; wherein said diffraction grating is arranged at a prescribed distance (D G) from a Littman-type tunable external resonator laser arrangement in which said movable mirror is configured to rotate around a pivot which is the intersection point of the surface of said movable mirror and the surface of the said diffraction grating, said prescribed distance (D G) being along a direction parallel to the normal direction to the grating surface in said Littman-type arrangement, so that mode-hop is suppressed, characterized in that the rear end surface mirror of said laser chip is moved by a predetermined offset Δ in a direction parallel to the emitted light from said laser chip, wherein the prescribed distance is equal to an offset D G of the pivot and the surface of said diffraction grating, said offset D G is determined by the following formula: D G = d 2 n 1 λ dλ 2 | λ 0 l + d 2 n 2 λ dλ 2 | λ 0 t 1 − λ 0 d − sin α 2 3 d 2 wherein: l 0 is desired center wavelength, n 1 ( λ) is effective refractive index of waveguide mode of laser chip, n 2 (λ) is refractive index of collimation lens, d 2 n 1 (λ) / dλ 2 or d 2 n 2 (λ) / dλ 2 is respectively quadratic differential coefficient for each wavelength of refractive index, l is the chip length of laser chip, t is effective thickness of collimation lens, d is interval of groove of diffraction grating, α is incident angle between collimated light and diffraction grating which collimating light is incident. l 0 is desired center wavelength, n 1 ( λ) is effective refractive index of waveguide mode of laser chip, n 2 (λ) is refractive index of collimation lens, d 2 n 1 (λ) / dλ 2 or d 2 n 2 (λ) / dλ 2 is respectively quadratic differential coefficient for each wavelength of refractive index, l is the chip length of laser chip, t is effective thickness of collimation lens, d is interval of groove of diffraction grating, α is incident angle between collimated light and diffraction grating which collimating light is incident.