PURPOSE: To obtain an interfacial membrane of stable quality by providing at least a detecting means detecting the height of interface in a vessel and a feedback mechanism to remove the fluctuation of interfacial pressure due to the height fluctuation of interface in addition to the ordinary mechanisms.

CONSTITUTION: For instance, a surface pressure meter, a water surface height detector using hang plates 3, 3' and a subtraction circuit subtracting the pressure fluctuation corresponding to the fluctuation of the height of water surface from the surface pressure acting as a feedback mechanism are constitute an LB- membrane forming device. A water vessel 1 is filled with pure water and the gas-liquid interface is cleaned, and in that state, the standard surface pressure is calibrated to be 0.00mN/m. Then, the apparent fluctuation of surface pressure due to the evaporation of water is measured at every 4hr for 48hr. As a result, the surface pressure meter by itself shows 0.09mN/m apparent surface pressure after 48hr while the water surface height detector shows 0.08mN/m. The real surface pressure obtained from the difference is 0.00W0.01mN/m during 48hr. Namely, a constant interface pressure is easily kept during the membrane production with the result an interfacial membrane of stable quality is obtained.