A tunable laser includes a resonator including a highly reflecting mirror and a partially reflecting mirror; a gain medium disposed in the resonator; a birefringence filter disposed in the resonator; and an excitation device that excites the gain medium. In the tunable laser, light that passes through the birefringence filter oscillates between the highly reflecting mirror and the partially reflecting mirror. An oscillation wavelength is switched by rotating the birefringence filter. The birefringence filter includes a first birefringent plate and a second birefringent plate, which have principal dielectric axes parallel to optical surfaces thereof. An absolute value of an angle formed by the principal dielectric axis of the first birefringent plate and the principal dielectric axis of the second birefringent plate is larger than zero.