PURPOSE: To compensate the rigidity or elastic change of a flexible tubing pressing out fluid when compressed and control fluid flow with high reliability in a valve system which controls fluid flow in a passage formed by a flexible member and is preferably applicable in a phleboclysis system.

CONSTITUTION: Components including pivot mounts 102, 104 pivotably supporting an inlet cracking valve and an outlet cracking valve 46, 52 respectively are mounted on a frame 100 in a housing 70 of a volumetric pump 30. Each cracking valve 46, 52 varies the contact pressure of each valve, face 106a, 106b with a flexible tubing 34 by means of pivoting. Fluid flow is controlled by pressing the flexible tubing 34 to a pressing plate 94. A valve arm 108 extends upwardly from the valve face 106a of the valve 46 and is rocked by the rotation of a rotating cam assembly 142. A tapered torque compensating roller 172 rotates in a torque compensating race 170 arranged at the back of the assembly 142, thereby applying rotation torque to the assembly 142.