To allow simple manufacture and control by joining two control portions relatively rotatable in a coaxial manner via a torsion bar with torsional deflection and friction-joining the torsion bar at its end to the corresponding valve control portion.

A servo valve 1 includes a fixed housing 2, a valve bush 3 for a valve control portion rotatable therein and a rotary valve element 4 for another valve control portion concentrical therewith therein. The rotary valve element 4 is rotatable in a limited range in the valve bush 3 and the valve bush 3 and the rotary valve element 4 are joined to each other via a torsion bar 5 attempting to hold the rotary valve element 4 in the valve bush 3 at a middle position. At least one end of the torsion bar 5 is friction-joined directly to valve control portions 3, 4 corresponding to the end or indirectly via a portion to be joined to the end in a relatively non-rotatable manner. In this arrangement, friction is caused by the elastic deformation of the friction-joined portion and the valve control portion 3 or 4 overcomes the friction, providing for control of the end of the torsion bar 5 corresponding thereto with the rotation. Simple manufacture and control are thus allowed.