More preferably, the thermoplastic copolymer A includes: as a maleimide unit, at least one type of maleimide unit in which R of formula (2) is an alkyl group selected from a methyl group, an ethyl group, an isopropyl group and a cyclohexyl group; and as an olefin unit, at least one type of olefin unit in which R1 in formula (1) is a hydrogen atom and R2 and R3 each are a methyl group.