PURPOSE: To form a heat-treated high-grade monomolecular film on the surface of a liq. by controlling a temp. changing means provided directly to the bottom plate of a through based on the temp. of the surface of the stored liq. detected by a temp. sensor.

CONSTITUTION: The temp. of a monomolecular film forming water surface 10 is controlled to a temp. set by a control panel 17, when a monomolecular film is set heat-treated. Plural thermomodules 4 are energized and controlled by the temp. control means 13 of a microcomputer 11 at the temp. detected by a temp. sensor 16 to directly change the temp. of the liq. in a through 2 and hence the temp. of the liq. surface through the bottom plate 2a of the through 2. A quick response is obtained since the depth of the liq. stored on the through 2 is very small. A movable partition plate 9 is moved so that the surface pressure difference between the water surface 10 and a second water surface 8 is adjusted to the set surface pressure to form a monomolecular film. The temp. of a temp. setting device is then changed to energize and control the plural thermomodules 4, and the monomolecular film is quickly heat-treated.