To provide a high-pressure fuel supply pump having a relief valve mechanism capable of suppressing deterioration of the seat property due to the influence of deformation caused by press-fitting while press-fitting and fixing a relief seat. To achieve this, a relief seat 201 of a relief valve mechanism 200 has, on its inner peripheral side, a seat portion 201a on which a valve 202 is seated, a small-diameter channel portion 201b formed with a smaller diameter than the valve 202 on an upstream side of the seat portion 201a, and a large-diameter channel portion 201c formed with a larger diameter than the small-diameter channel portion 201b on an upstream side of the small-diameter channel portion 201b, and, on its outer peripheral side, a fine gap portion 201d formed between the relief seat 201 and a member arranged on an outer peripheral side of the relief seat 201 at a position overlapping the small-diameter channel portion 201b in a flow direction of the fuel, and a press-fit portion 205a coming into contact with the member when the relief seat 201 is press-fitted into the member at a position overlapping the large-diameter channel portion 201c in the flow direction of the fuel.