An apparatus for producing monomolecular films or monomolecular built-up films includes: a trough for accommodating a liquid over the surface of which a monomolecular film is to be spread; a barrier band for defining a region in the trough over which the monomolecular film is allowed to be spread; and a plurality of supports for holding the barrier band within the trough, the supports being arranged at positions where they do not interfere with the surface of the liquid, wherein the loss of the monomolecular film material can be substantially reduced. Further, no local collapse of the molecular film occurs in the vicinity of the supports, thereby making it possible to always provide a uniform LB film.