A filter to be mounted in a valve seat member for filtering liquid is proposed which can be manufactured without a drawing step, thereby improving productivity and cost performance of a piston pump in which such a filter is mounted. The filter is a flat metal plate including a main body formed with a plurality of filter holes in a central portion thereof, and a plurality of projections integrally and radially outwardly extending from a perimeter of the main body so as to be circumferentially spaced apart from each other. The filter is pushed into a filter mounting hole formed in a valve seat member of a discharge valve in a piston pump with the projections bent toward the opening of the filter mounting hole while being pressed against the inner surface of the filter mounting hole under an elastic restoring force and fixed to the valve seat member.