Provided is a graft copolymer (A) obtainable by graft-polymerizing, in the presence of 30 to 80 parts by mass of an ethylene-based polymer (a), an aromatic vinyl-based monomer (b) and, if necessary, another vinyl-based monomer (c) copolymerizable with the aromatic vinyl-based monomer (b) in a total amount of 20 to 70 parts by mass (provided that the sum of the components (a), (b) and (c) is 100 parts by mass), in which the ethylene-based polymer (a) has a density of 0.900 g/cm3 or more and less than 0.966 g/cm3, a number average molecular weight of 10,000 to 50,000, and a molecular weight distribution of 5 to 15, and the proportion of the ethylene-based polymer (a) molecules having a molecular weight of 10,000 or less is 5.5% or more.