PURPOSE: To provide a controlling method of interfacial roughness, which facilitates the setting of forming conditions, under which no interfacial roughness develops even in a plurality of resin of any kinds.

CONSTITUTION: A plurality of the kinds of the combination of resins different from one another are selected. Next, the first interfacial normal stress difference, under which interfacial roughness develops, is obtained through experiment to the change of the predetermined resin characteristics in respective combinations. After that, on the basis of the distribution of the first interfacial normal stress differences to the predetermined resin characteristics, the critical range of the first interfacial normal stress difference for deciding the presence or absence of the development of the interfacial roughness is obtained in order to change forming conditions so as to be brought into the critical range on the combination of the resins to be estimated.