A stabilizer containing a copolymer (A) obtained by polymerizing an unsaturated monomer mixture including 1 to 50 parts by mass of ethylenically unsaturated monomer (a) (formula (I)) having a piperidyl group in a molecule, 50 to 99 parts by mass of at least one monomer (b) (isobutyl methacrylate, an alkyl (meth)acrylate having an alkyl group with 6 to 13 carbon atoms, or an aromatic vinyl monomer), and 0 to 20 parts by mass of ethylenically unsaturated monomer (c) other than the monomers (a) and (b), where the total of the monomers (a), (b), and (c) is 100 parts by mass: wherein R1 represents H or an alkyl group having 1 to 2 carbon atoms, X represents an oxygen atom or an imino group, Y represents H or an alkyl or alkoxyl group each having 1 to 20 carbon atoms, and Z represents H or a cyano group.