To obtain a polymer for photoetching, having remarkable contract and excellent in thermal characteristics by including a polymer obtained from plural (i.e., larger than a specific number) kinds of monomers and combined in its polymeric skeleton with a specific group causing a chemical changed due to an acid.

This polymer in obtained by polymerizing plural (i.e., larger than three) kinds of monomers and by combining the polymeric skeleton of the resultant polymer with a group causing a chemical change due to an acid which is expressed by the formula CH(COOR1)2 (wherein, R1 is a hydrocarbon group or its derivative group) and pref. di-t-butyl malonate group or di- tetrahydropyranyl malonate group. The polymer has a weight average molecular weight 5,000-200,000 and is pref. expressed by formula I [wherein, R1 is t-butyl or the like; R4 is methyl or the like; R2, R3, R5 and R6 are each H, methyl, or the like; (l), (m) and (n) are an integer; (l)/((l)+(m)+(n))=0.01-0.5; (m)/((l)+(m)+(n))=0.01-0.5; (l)+(m)/((l)+(m)+(n))=0.1-0.5] or formula II (wherein, R7 is t-butyl or the like; R8 is methyl or the like). This composition is prepared by including the polymer.