PURPOSE: To obtain comparatively high picture quality by giving the pump such a characteristic as to work as a constant pressure pump at the time of rising after start and as a constant flow pump under the steady condition and thereby suppressing variation in the level of printing.

CONSTITUTION: Because discharge pressure of the pump's pressure chamber 6 is small at the time of rising for inking, the pushing force of a spring 7 becomes dominating, and a piston 5 reciprocates in the condition that a plunger 1 is protruding the most. When thereafter the discharge pressure has increased, the piston 5 under discharging action intrudes into the plunger 1 till a position where the discharge pressure is in balance with the pushing force of spring 7. When further the discharge pressure increases to get a steady condition, the piston 5 under discharge action will take a position as the deepest inside the plunger 1. Thus the pump presents a characteristic that the pressure rise is rapid in the range below a certain specific pressure at the time of rising and that the work in the steady condition is a one of constant flow pump, so that variation in the level of printing is small to assure a high picture quality.