PURPOSE: To avoid making a pump casing enlarged in size by reducing the outside dimensions of a bearing as small as possible at the time of constituting the bearing to be made in contact with a plunger with a ball bearing which produces almost no thrust force and thereby dispensing with any members including a bush and the like absorbing the thrust force.

CONSTITUTION: When a bearing is fitted in the outer boundary of an eccentric shaft part 4a being formed in a motor shaft 4 and this bearing is made contact with the end face of a plunger 10, this bearing is composed of a pair of ball bearings 11 receiving this plunger load in a dispersed manner, and thereby bearing outside dimensions (d) are made so as to be reduced as small as possible in terms of constitution.