The polymer which designates amphoteric monomer and the cation characteristic monomer of specification which are displayed with below-mentioned general formula (i) as necessary constitution monomer. (In formula, as for R1 as for hydrogen atom or methyl group and R2 and R3 becoming independent respectively, carbon count the alkyl group or alkoxy group and carbon count shikuroarukiru of 3 of 1 - 4 - 6, phenyl group, or - (CH2 CH2 O) the nR7 basis (as for n integer of 1 - 50, as for R7 hydrogen and carbon count the alkyl group or alkoxy group and carbon count you display shikuroarukiru, or phenyl group of 3 of 1 - 4 - 6), as for R4 carbon count the alkylene basis of 1 - 3, as for A oxygen atom or NR8 (as for R8 the hydrogen atom or carbon count alkyl group or alkoxy group 1 - 4,Phenyl group, or - (CH2 CH2 O) the nR9 basis (as for n integer of 1 - 50, as for R9 hydrogen and carbon count the alkyl group or alkoxy group and carbon count shikuroarukiru, or phenyl group 3 of 1 - 4 - 6 is displayed))So basis and B which are displayed carbon count show the alkylene basis 1 - 4.)下記一般式(I)で表される両性モノマーと、特定のカチオン性モノマーとを必須構成モノマーとするポリマー。(式中、R1は水素原子又はメチル基、R2及びR3はそれぞれ独立して炭素数1~4のアルキル基あるいはアルコキシ基、炭素数3~6のシクロアルキル基、フェニル基、又は-(CH2CH2O)nR7基(nは1~50の整数、R7は水素、炭素数1~4のアルキル基あるいはアルコキシ基、炭素数3~6のシクロアルキル基、又はフェニル基を表す)、R4は炭素数1~3のアルキレン基、Aは酸素原子又はNR8(R8は水素原子又は炭素数1~4のアルキル基あるいはアルコキシ基、フェニル基、又は-(CH2CH2O)nR9基(nは1~50の整数、R9は水素、炭素数1~4のアルキル基あるいはアルコキシ基、炭素数3~6のシクロアルキル基、又はフェニル基を表す))で表される基、Bは炭素数1~4のアルキレン基を示す。)
