To easily wet a filter with water, easily and precisely measure the perfectness of the filter, and raise the reliability by adding a cross linking monomer to holes in both end parts of a cylindrical pleated membrane, carrying out polymerization, and losing ventilative property.

A cross linking monomer is added to holes in both end parts of a cylindrical pleated membrane and the monomer is polymerized to lose ventilative property. As the cross-linking monomer material, hydrophilic vinyl monomer, acrylates and methacrylates type monomer are preferably used. A solution produced by adding solely these monomers or their mixtures additionally mixed with multifunctional vinyl monomer and an initiator is applied to both end parts of a membrane and polymerization reaction is carried out to form hydrophilic polymer in the membrane holes. The concentration of the solution to be applied is preferably about 10-50wt.%. On the other hand, the necessary buried amount of the polymer to bury the holes of the membrane and lose the ventilation is at the lowest about 1mg/cm2 per unit surface area of the membrane.