Organic groups represented by R1 to R5 and Z1 to Z6 other than that having no water-solubilizing group are each selected from a hydrogen atom, an alkyl group such as a methyl group and an ethyl group, a substituted alkyl group, an alkenyl group such as an allyl group, a substituted alkenyl group, an aryl group such as a phenyl group and a substituted aryl group such as a p-tolyl group.