The invention aims to provide a method for preparing a composite substrate, which prevents an insulating layer surface from becoming rugged under the influence of an electrode layer, and eliminates a polishing step, whereby the composite substrate is easy to manufacture and ensures high display quality when applied to thin-film light emitting devices, the composite substrate and an thin-film EL device using the same. The object is attained by a method for preparing a composite substrate by successively applying an electrode paste and an insulator paste onto an electrically insulating substrate as thick films to form a composite substrate precursor having a green electrode layer and a green insulator layer laminated, subjecting the precursor to pressing treatment for smoothing its surface, and firing; the composite substrate and an EL device using the same.