To provide a composite membrane composed of a nonwoven fabric integrated with a porous thin film containing a pore group having high pore size uniformity and high pore ratio suitable for the separation of a micron-sized object and useful as a separate-type cell culture substrate and a membrane for the coculture of multiple kinds of cells because various cells are easily introducible into the nonwoven fabric or further to the surface of the porous thin film on the nonwoven fabric.

The composite membrane contains at least one porous thin film containing an organic polymer compound and at least one nonwoven fabric placed adjacent to the porous thin film, wherein the organic polymer compound constituting the porous thin film is intruded into the nonwoven fabric; the porous thin film has specific pore ratio, average pore diameter, standard deviation σd of the pore diameter, penetration ratio of the pore, average film thickness and internal structure; and the nonwoven fabric has a structure containing at least one kind of thin fiber having an average fiber diameter of 7-30 μm and at least one kind of fine fiber having an average fiber diameter of 0.5-5 μm and blended with the thin fiber in an interlaced state.

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