A heat treatment apparatus for a selenization process or a sulphurization process carried out when forming a light absorbing layer in a chalcopyrite-type solar cell, comprises a quartz tube in which a plurality of solar cell substrates is arranged in parallel at predetermined intervals in a thickness direction, a heating mechanism for heating atmospheric gas, which is arranged at an outside of the quartz tube, and first baffle plates arranged upward of the substrates, in which heated atmospheric gas, which rises along an inner surface of the quartz tube, is guided from upward to the center of the substrates.