NOVELTY - The mechanism (80) has a ball (81) abutted with a wall face (64c) of a rack section (63). A spring (82) presses the ball toward the wall face to generate a frictional force to retain a valve body (46) at a predetermined opening position. A retainer (83) includes a retaining face (84) facing in one direction along an axial direction toward a spring side to retain the ball at the retaining face. A spring receiving face (88) is faced in the former direction for receiving a biasing force in another direction of the spring.    USE - Locking mechanism for a valve device (claimed) i.e. fluid controlling valve device, incorporated in a fuel cell system.    ADVANTAGE - The ball is pressed against the wall face of the rack section by the spring member, so that a frictional force is generated between the wall face of the rack section and the ball, and the rack section is retained by the frictional force so as to retain the valve body at the predetermined opening position even if the rotational driving source is not energized, thus reducing the power consumption and eliminating the need for a special driving mechanism to cancel the retention of the valve body at the predetermined opening position. The mechanism distributes the biasing force of the spring to multiple balls by the abutting face, so that a frictional force is generated at a number of locations equal to the number of balls between the wall face of the rack section and the balls, thus retaining the rack section with a higher degree of certainty when the wall face of the rack section is retained stably at multiple locations. The mechanism has a simple structure and reduced size and weight, thus reducing cost of the mechanism.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a valve device.    DESCRIPTION OF DRAWING(S) - The drawing shows a side sectional view of a valve device incorporated in a fuel cell system.    Valve body (46)    Rack section (63)    Wall face (64c)    Locking mechanism (80)    Ball (81)    Spring (82)    Retainer (83)    Retaining face (84)    Spring receiving face (88)