A paper feeder equipped with copying paper cassettes comprises a cassette-receiving section within the housing of a copying apparatus for mounting thereon a plurality of copying paper cassettes containing sheets of copying paper having different sizes. Each of the paper cassettes includes a main body for holding paper sheets which has an opening formed at the forward end portion of its bottom wall and a bottom plate for placing paper sheets thereon which is mounted on the bottom wall of the main body of the cassette so that it can freely pivot about its rear end portion as a fulcrum. The housing of the coyping apparatus has provided therein a press-contacting mechanism adapted to act on the bottom plate of the cassette through the opening in the main cassette body. The bottom plate of at least that paper cassete which contains sheets of copying paper having a relatively large size has a compensation piece formed therein so that the press-contacting pressure under which the uppermost paper sheet of the sheets of paper having a relatively large size is kept in press contact with a paper feeding member provided in the cassette-receiving section becomes substantially equal to the press-contacting pressure under which the uppermost paper sheet of the sheets of copying paper having a relatively small size is kept in press contact with the paper feeding member.