To obtain a copolymer having permeability to water vapor but impermeable to liquid water by using ethylene, a polyalkylene glycol (ether) (meth) acrylate monomer, etc., as the monomer units.

This copolymer is composed of (A) 99-50wt.% of ethylene, (B) 1-50wt.% of a monomer expressed by the formula (R1 is H or CH3; (m) is 2-4; (n) is 2-25), e.g. polyethylene glycol (ether) (meth)acrylate and (C) 0-20wt.% of a third monomer and has an average molecular weight of 5,000-65,000. Preferably, the copolymer has a melt index MI of 0.5-100 and the component C is e.g. an α-olefin having one or plural unsaturated groups and at least 3 carbon atoms.