In a high pressure fuel supply pump for transmitting rotation of a cam to a reciprocating plunger via a tappet and a retainer, a diametric force acting on the plunger is reduced. The pump includes the retainer disposed on the plunger and a return spring exerting an urging force on the retainer in a direction of the tappet. A clearance between a plunger leading end and a tappet bottom surface opposed thereto is set to be greater than a clearance between a retainer bottom surface and the tappet bottom surface opposed thereto, and a clearance between a retainer inside diameter section and a plunger peripheral surface section opposed thereto is set to be greater than a clearance between a retainer outside diameter section and a tappet inner wall opposed thereto.