The method of manufacturing a light absorbing layer for a
solar cell by performing
thermal treatment on a specimen configured to include thin films of one or more of
copper,
indium, and
gallium on a substrate and element
selenium, includes steps of: (a) heating a wall of a chamber up to a predefined thin film formation temperature in order to maintain a
selenium vapor pressure; (b) mounting the specimen and the element
selenium on the
susceptor at the
room temperature and loading the
susceptor in the chamber; and (c) heating the specimen in the lower portion of the
susceptor and, at the same time, heating the element selenium in the upper portion of the susceptor, wherein, in the step (c), in order for liquefied selenium not to be condensed on the specimen which is loaded at the
room temperature and is not yet heated, the temperature of the element selenium and the specimen loaded in the chamber are individually controlled, so that the selenium
vapor pressure of an inner space of the chamber does not exceed a saturation
vapor pressure corresponding to the temperature of the specimen.