The present invention addresses the problem of providing: a recycled styrene monomer which is a recycled product from a waste polystyrene product and is such that energy consumption during production is reduced; a styrene-based resin using the recycled styrene monomer; a styrene-(meth)acrylic acid-based copolymer; a polymer alloy; a composition; a sheet; a film; a laminate; a molded body; and a production method for a polymer using the recycled styrene monomer.　The problem is solved by: a recycled styrene monomer which contains styrene and is a product obtained by subjecting a waste polystyrene product to a pyrolysis treatment; and a production method for a polymer which involves polymerizing the starting materials for a polymer containing said recycled styrene monomer.