PURPOSE: To improve workability of the retainer of a servo device when the base of a valve element is fixed on the internal peripheral part of the valve body in comparison with the conventional case.

CONSTITUTION: A retainer 13 for fixing the base 11a of a valve element 11 to a step part 2a is provided with an annular groove A for holding the base 11a of the valve element 11, an inner cylindrical part 13c and an outer cylindrical part 13d formed on the rear side of the annular groove A, and the front side end part of the outer cylindrical part 13d is made an engaging part 13e in contact with the step part end surface of the step part 2a. In such constitution, when the valve element 11 and the retainer 13 are fitted in and assembled to the peripheral part of a valve body 2 from the rear side, the outer cylindrical part 13d smoothly slides on the inner peripheral part of the valve body 2 so as to achieve the function of a guide. Accordingly, the valve element 11 and the retainer 13 are smoothly fitted to the position of a step part end surface so that the assembling thereof can be completed.