PURPOSE: To easily measure an interface energy between two kinds of molten polymers by discharging a multi-layer point composite polymer in which more than 32 of layers of two different kinds of polymers are alternately joined together, from a discharge hole having a predetermined cross-sectional shape, and by calculating the interface energy between the two kinds of polymers from the degree of difference between the cross-sectional shape of a thread-like substance and the cross-sectional shape of the discharge hole.

CONSTITUTION: A multi-layer composite polymer stream in which more than 32 of two different kinds of polymers are alternately and repeatedly joined together is discharged from a discharge hole having a circular or square cross-sectional shape, and accordingly, the cross-sectional shape of the stream discharged from the discharge hole has a multi-layer structure having interfaces which are parallel with each other, and is flat due to interface energy. The degree of flatness of the cross-section is exhibited as C=A/B where A is the axial length in a direction orthogonal to the interfaces in the cross-section and B is the axial length in a direction parallel with the interfaces. Further, the outer periphery G of the cross-section and the total length L of the interfaces may be calculated from the total discharge volume and discharge velocity of the polymer by approximation to elliptic or rectangular shape. The outer periphery G and the total distance L are automatically determined from the degree C of flatness since the sectional shape is constant if the discharge volume of the polymer is constant.