PURPOSE: To adjust stroking of plunger pump in stepless by changing the contact face position of roller contacting with cam face of cam cylinder coupled to a motor thereby varying stroke of stroke rod coupled to the roller.

CONSTITUTION: Upon rotation of motor 2, a gear 9 geared with a screw cylinder 4 fixed on a rotary shaft 2a will rotate together with a cam cylinder 8 fixed to it. Consequently a roller 14 face contacting with the cam cylinder 8 will function a stroke rod 18 through L-shaped links 15, 16 to hit on the main shaft 3a of plunger pump 3 to perform pumping. If a support 5 is moved through operation of screw rod 7, the contact face position between the cam face 8a of cam cylinder 8 and roller 14 will vary in stepless from (a) to (c), to vary stroke of stroke rod 18 through the links 15, 16 resulting in stepless adjusting of operation of plunger pump 3.