PURPOSE: To obtain a composite membrane not having defects such as the formation of minute faults and non-uniform membrane thickness, by guiding a porous support coated with a polymerization initiator into a monomer atomizing atmosphere.

CONSTITUTION: A hollow yarn having an outer diameter of 200W1,000μm, an inner diameter of 160W800μm, a void ratio of about 30W80% on the basis of a surface area and an average pore size of 0.05W0.3μm is used as a support. 0.1W 0.5wt% of a polymerization initiator is applied to the support to perform coating. Thin membrane forming monomer is brought to a high density aerosol state, wherein 107W108/cm2 of mist droplets with a particle size of 3μm or less are distributed, by using an ultrasonic vibrator or air gun and the support coated with the polymerization initiator is guided to be passed through this atomizing atmosphere. After the support was contacted with said atmosphere for 30secW2min, the coated support is dried and a composite membrane uniformly coated and having a membrane thickness of 1μm or less can be obtained.