It is intended to prevent ink from leaking through the ink outlet of the ink rail while the printing press is at halt. To achieve this object, a motor control section 40 shown in FIG. 4 controls the rotation of a stepping motor 15 driving a plunger 13 and, every time the stepping motor 15 is stopped, the outer circumference 13c of the plunger 13 except a cut portion 13a blocks at least a discharge port 18, thereby enabling the pressure ink invading from an intake port 17 into the main hole 11 of a cylinder 12 to press the plunger 13 against the discharge port 18 opening into the inner circumferential face of the main hole 11.