PURPOSE: To increase a water permeating speed while excellent characteristics of a polyimide film is maintained by introducing a specific amount of a carboxyl group into the polymer chain of aliphatic polyimide comprising a specific structural unit as a side chain.

CONSTITUTION: An imide unit shown by formulaIand a carboxylated imide unit shown by formula II are dissolved in N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide or a mixed solvent thereof along with 10W30pts. of swelling agent per 100pts. of polyimide to form a uniform dope with viscosity of 50W300poise. This dope is coated on a plate and a pipe having a smooth surface such as glass or the like or a base material of a fibrous sheet or a pipe and, after a part of the solvent is evaporated, the coated material is immersed in a coagulating solvent to coagulate polyimide. Thus obtained film has heat resistance, solvent resistance, chemical resistance, anti-bacterial property as well as a high water permeating speed.