NOVELTY - An element of group IIIB and an element of group VIB are vapor-deposited to form thin film (a) on a substrate (2). An element of group IB and an element of group VIB are vapor-deposited on thin film (a), to form thin film (b). An element of group IIIB and an element of group VIB are vapor-deposited on thin film (b), to form thin film (c). The predicted value is calculated for composition ratio of element of group IB and element of group IIIB in thin film at end point of film forming zone (11-13) (where position detection is carried out), based on the detected position.    USE - Formation of film e.g. copper-indium-gallium-selenium film used for solar cell.    ADVANTAGE - The method enables formation of film by controlling composition of vapor-deposited compounds on the substrate.    DETAILED DESCRIPTION - An element of group IIIB and an element of group VIB are vapor-deposited to form thin film (a) on a substrate. An element of group IB and an element of group VIB are vapor-deposited on the obtained thin film (a), to form thin film (b) containing composition with excess of element of group IB with respect to stoichiometric composition ratio of element of group IB, element of group IIIB and element of group VIB. An element of group IIIB and an element of group VIB are vapor-deposited on thin film (b), to form thin film (c) containing composition with excess of element of group IIIB with respect to stoichiometric composition ratio. A position detection unit detects position of substrate at which composition ratio of element of group IB, element of group IIIB and element of group VIB becomes stoichiometric composition ratio, during formation of thin film (b). The predicted value is calculated for composition ratio of element of group IB and element of group IIIB in thin film at end point of film forming zone (where position detection is carried out), based on the detected position. The vapor deposition amount of element of group IB during formation of thin film (b) is controlled such that the composition ratio element of group IB, element of group IIIB and element of group VIB in thin film (b) is preset intended value of composition ratio of element of group IB and element of group IIIB of thin film containing element of group IB, element of group IIIB and element of group VIB in end point of formation of thin film (b), based on predicted value. The thin film containing element of group IB, element of group IIIB and element of group VIB is formed on substrate using an in-line system. An INDEPENDENT CLAIM is included for film forming apparatus (1).    DESCRIPTION OF DRAWING(S) - The drawing shows the block diagram of the film forming apparatus. (Drawing includes non-English language text)    Film forming apparatus (1)    Substrate (2)    Reaction chamber (3)    Preheating chamber (6)    Film forming zones (11-13)