A torsion spring that is used in a throttle valve device increasing and decreasing an opening degree of an intake passage or an exhaust passage of an internal-combustion engine includes: two coil springs connected with each other so that torsion directions are opposite from each other; and a hook disposed between the two coil springs. Of the two coil springs, one side spring arranged at one side in an axial direction biases a valve object to a closing side, and the other side spring arranged at the other side in the axial direction biases the valve object to an opening side. In at least one coil spring of the two coil springs, a dimension of a clearance between the at least one coil spring and the hook connected with each other is larger than an average value of clearances between coil turns of the at least one coil spring.