A tunable laser source of a Littman arrangement type has a wavelength selection mirror rotating around a position where a mode hop is suppressed in a wavelength variation, a feed screw receiving a driving force of a motor at one end to be rotated, in which another end is disposed at a predetermined position, and which is larger in coefficient of thermal expansion than the optical base, a first nut feedably fitted to the feed screw, a flat spring in which one end is fixed to the first nut; and a mirror arm having a rotation shaft disposed at a rotation center of the wavelength selection mirror, and first and second arm portions formed in a V-like shape and elongating from the rotation shaft. The wavelength selection mirror is fixed to the first arm portion, and another end of the flat spring is fixed to the second arm portion.