The present invention relates to an improved clutch booster equipped with a power piston and a control valve unit, and more particularly to a clutch booster wherein the control valve is operatively connected to a clutch pedal by mechanical link means and there is employed at least one pivot lever as an essential element of the link means, and wherein the pivot lever is pivotally connected to an output rod in such a manner that a manual force rendered upon the clutch pedal of a vehicle may be relayed in the directions of the output rod, thereby enabling the manual clutch operation even in the non-servo operation state where the clutch booster is left not supplied with any operating fluid pressure, and thereby eliminating the need for such complicated mechanical installation as a master cylinder, fluid pipings, etc. incorporated in the connection between the clutch pedal and the clutch booster.