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The resin (A3) has a group which is capable of decom› posing to become alkali-soluble by the action of an acid (acid-decomposable group). In this arrangement, when acted on by an acid, the resin (A3) decomposes to increase in its solubility in an alkaline developer. The acid-decomposable group may be possessed by the repeating unit having a group represented by the formula (Z), maybe Di’, D$_{1 }$ or D$_{2 }$ in the repeating units represented by the formulae (Illa), (Illb) and (IVb) or may be possessed by other repeating units. 

Examples of the acid-decomposable group include -O-C(R 19 )(R 20 )(R 21 ), -O-C(R 19 )(R 20 )(OR$_{22}$ ), -O-COO-C(R 19 )(R 20 )(R 21 ), -O-C(R 01 )(R$_{02}$)COO- C(R 19)(R20)(R21), -COO-C(R 19 )(R 20 )(R 21 ), and 65 -COO-C(R 19 )(R 20 )(OR 22 ). R 01 and R$_{02 }$ each indepen› dently represent a hydrogen atom or an alkyl, cycloalkyl, 

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alkenyl, aralkyl or aryl group which may have substituents. R$_{19 }$ to R$_{22 }$ may be the same or different and each represent an alkyl, cycloalkyl, alkenyl, aralkyl or aryl group, which may have a substituent. Two of R$_{19}$ , R$_{20 }$ and R$_{21 }$ or two of R$_{19}$ , R$_{20 }$ and R$_{22 }$ may be bonded to each other to form a ring. 

The aforementioned alkyl group is preferably a CcC$_{8 }$ alkyl group. Examples of such an alkyl group include methyl group, ethyl group, propyl group, n-butyl group, sec-butyl group, hexyl group, 2-ethylhexyl group, and octyl group. 

The cycloalkyl group may be monocyclic or polycyclic. Preferred examples of the monocyclic cycloalkyl group include C$_{3}$-C$_{8 }$ cycloalkyl groups. Examples of the cycloalkyl group include cyclopropyl group, cyclopentyl group, cyclohexyl group, cyclobutyl group, and cyclooctyl group, Preferred examples of the polycyclic cycloalkyl group include C$_{6}$-C 20 cycloalkyl groups. Examples of the cycloalkyl group include adamanthyl group, norbornyl group, isoboronyl group, camphanyl group, dicyclopentyl group, a-pinene group, tricyclodecanyl group, tetracy- clododecyl group, and androstanyl group. 

Preferred examples of the alkenyl groups include C$_{2}$-C$_{8 }$ alkenyl groups. Examples of the alkenyl group include vinyl group, allyl group, butenyl group, and cyclohexenyl group. 

Preferred examples of the aryl groups include C$_{6}$-C$_{10 }$aryl groups. Examples of the aryl group include phenyl group, tolyl group, dimethylphenyl group, 2,4,6-trimethylphenyl group, naphthyl group, anthryl group, and 9,10- dimethoxyanthryl group. 

Preferred examples of the aralkyl groups include C$_{7}$-C$_{12 }$ aralkyl groups. Examples of the aralkyl group include ben› zyl group, phenethyl group, and naphthylmethyl group. 

An example of the ring formed by the connection of two of R$_{19 }$ to R$_{22 }$ is a 3-membered to 8-membered ring. Specific examples of such a ring include cyclopropane ring, cyclo- pentane ring, cyclohexane ring, furane ring, pyrane ring, dioxonol ring, and 1,3-dioxolane ring. 

Examples of the substituents on these groups include those having an active hydrogen such as amino group, amide group, ureide group, urethane group, hydroxyl group and carboxyl group, halogen atoms (e.g., fluorine atom, chlorine atom, bromine atom, iodine atom), alkoxy groups (e.g., methoxy group, ethoxy group, propoxy group, butoxy group), thioether groups, acyl groups (e.g., acetyl group, propanoyl group, butoxy group), acyloxy groups ( e.g., acetoxy group, propanoyl group, benzoyl group), alkoxy› carbonyl groups (e.g., methoxycarbonyl group, ethoxycar› bonyl group, propoxycarbonyl group), cyano groups, and nitro groups. 

Specific preferred examples of the acid-decomposable group include tertiary alkyl ether or ester groups such as t-butyl group, 5-amyl group, l-alkyl-1-cyclohexyl group, 2-alkyl-2-adamanthyl group, 2-adamanthyl-2-propyl group and 2-( 4-methylcyclohexyl)-2-propyl group, acetal or acetal ester groups such as l-alkoxy-1-ethoxy group and tetrahy- dropyranyl group, t-alkylcarbonate groups, and t-alkylcarbonylmethoxy groups. Particularly preferred among these acid-decomposable groups are acetal groups such as l-alkoxy-1-ethoxy group and tetrahydropyranyl group. 

These acetal groups are advantageous in that they exhibit a great acid-decomposability that increases the range of selection of the acid-producing compounds to be used in combination therewith, making it possible to enhance the sensitivity of the composition and inhibit the change of properties of the composition with time between exposure and heating. Particularly preferred is an acetal group con- taining as 1-alkoxy component of acetal group an alkoxy 

