One embodiment of the present invention is a film forming method including the steps of forming an absorption layer 12 over one surface of a first substrate 11; forming a layer 16 containing a high molecular compound over the absorption layer; removing an impurity in the layer containing the high molecular compound by performing a first heat treatment on the layer 16; forming a material layer 18 containing a first film formation material and a second film formation material over the layer 16; performing a second heat treatment to form a mixed layer 19 in which the material layer and the layer 16 are mixed over the absorption layer; and performing third heat treatment to form a layer 19a containing the first film formation material and the second film formation material on a film-formation target surface of a second substrate.