To provide high conductivity, excellent electrolyte resistance and a good binding property by polymerizing and containing two kinds of specific (meta)acrylic acid alkyl ester, ethylene unsaturated carboxylic acid and a specific quantity of another monomer copolymerizable with them as required at specific ratios under the existence of a specific quantity of organo-polysiloxane.

Polysiloxane 1-90 pts.wt. and monomer components 99-10 pts.wt. in total are polymerized. The monomers are constituted of (meta)acrylic acid alkyl ester 1-70 wt.% having the carbon number 1-3 of the alkyl group, (meta) acrylic acid alkyl ester unit 25-97 wt.% having the carbon number 4-10 of the alkyl group, ethylene unsaturated carboxylic acid unit, and another monomer unit copolymerizable with them as required at 100 wt.% in total. The organopolysiloxane having a cyclic structure is preferably used, and methyl ester, ethyl ester or n-butyl ester is used for the (meta)acrylic acid ester.