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The presence of a substituent (Rb$_{2}$ ) on the portion of the lactone structure is optional. As a preferred substituent (Rb$_{2}$ ), there can be mentioned an alkyl group having 1 to 8 carbon atoms, a monovalent aliphatic hydrocarbon ring group having 4 to 7 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, an alkoxycarbonyl group having 1 to 8 carbon atoms, a carboxyl group, a halogen atom, a hydroxyl group, a cyano group, an acid-decomposable group or the like. Of these, an alkyl group having 1 to 4 carbon atoms, a cyano group and an acid-decomposable group are more preferred. In the formu› lae, n$_{2 }$ is an integer of O to 4. When n$_{2 }$ is 2 or greater, the plurality of present substituents (Rb$_{2}$ ) may be identical to or different from each other. Further, the plurality of present substituents (Rb$_{2}$ ) may be bonded with each other to thereby form a ring. 

The repeating unit having a lactone group is generally present in the form of optical isomers. Any of the optical isomers may be used. It is both appropriate to use a single type of optical isomer alone and to use a plurality of optical iso› mers in the form of a mixture. When a single type of optical 

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