PURPOSE: To treat uniformly a membraneous polymer without forming wrinkles and crazings, by laying the membraneous polymer on a knit of a synthetic fiber and treating it by contact with a gas or a liquid.

CONSTITUTION: In treating an ion exhange membrane, a diaphragm, or other membraneous polymers by contact with a gas and/or a liquid, the membraneous polymer is laid on a knit of a synthetic fiber. The knits used may be any of weft-knitted fabrics, warp-knitted fabrics, and tubular knitted fabrics, but those knitted fabrics which are knitted on a knitting machine set at a stitch determined from the equation (wherein GG is in gauge/inch, Td is a total denier of a yarn, and 1.2≤α≤2.4) are the most suitable. When α>2.4, the stitch is too dense, it is difficult to spread the treating solution over the surface of a film, and the treatment can not be performed evenly. When α<1.2, the stitch is too rough, the durability is insufficient and the treatment becomes uneven.