The invention provides fuel supply pump that can improve the durability of a fuel inlet valve and stably pressure-feed high pressure fuel of a large flow rate. In the fuel inlet valve according to the invention, a valve piston includes, in its outer surface in the vicinity of an end portion on an opposite side of a collar portion, a groove portion to which a fastener for fixing the valve piston and a spring seat locks. The spring seat includes an open portion into which the valve piston is inserted and includes, along an edge of the open portion, a step portion to which the fastener locks. The valve piston is inserted into the open portion of the spring seat, and the fastener is thereafter caused to lock to the groove portion of the valve piston. The spring seat is energized by a spring to cause the fastener to lock to the step portion, whereby the spring seat is fixed to the valve piston. A clearance is disposed between the fastener and at least one of an edge portion of the groove portion on an energizing direction side and an edge portion of the step portion on the opposite side of the energizing direction.