To provide a cycloolefinic resin composition which is capable of forming a crosslink or a quasi-crosslink due to a carboxylic group generated in the molecule of a commercially producible cycloolefinic addition polymer under a specific condition.

The cycloolefinic resin composition contains component A which is a cycloolefinic addition polymer obtained by addition-polymerizing a cycloolefinic monomer having a -(CHR2)k-C(O)OR1 group (wherein, R1 is a trialkylsilyl group containing at least one 1-15C alkyl group, cycloalkyl group or aryl group or 2-10C alkyl group; R2 is a hydrogen atom, a 1-10C alkyl group, a cycloalkyl group or an aryl group; and k is an integer of 0-5) and a cycloolefinic monomer not having such a group and component B which is a thermo acid generator generating an acid at ≥50°C. The method of treatment of the molding comprises bringing the molding containing the resin composition into contact with water or steam with a temperature of ≥90°C. The film and the sheet are obtained by the method.