This invention provides a method for forming a light absorbing layer in a CIS-based thin film solar battery, which, in a simple apparatus, the temperature within the apparatus can be rendered uniform and can improve contact between a reactive gas and selenium and sulfur. A fan (3) as an atmosphere homogenization means is provided within the apparatus. Further, objects are arranged by a method which can render the convection of the reactive gas smooth. Specifically, a plurality of flat-plate objects (2) are arranged while providing given spacing between the objects in such a manner that the objects are parallel to the longitudinal direction of the apparatus with the plate face being perpendicular thereto. Further, an internal flow passage is provided within the group of objects, and a gas flow passage is provided on the upper part, the lower part, and both sides. According to the above construction, each object can easily come into contact with the reactive gas within the apparatus, the temperature within the apparatus becomes uniform, and the contact between the reactive gas and selenium and sulfur is improved.