PROBLEM TO BE SOLVED: To provide a method for effectively removing a flame retardant comprising HBCD (hexabromocyclododecane) added into a molded material of foamed polystyrene particularly used as a building material.SOLUTION: A method for removing a flame retardant includes preparing a solvent mixture of a first solvent which does not dissolve polystyrene but dissolves HBCD and a second solvent dissolving polystyrene, impregnating the solvent mixture with foamed polystyrene, and after that, repeating one or plural times the operation which agitates and crushes a block of foamed polystyrene to remove the flame retardant. Thereby, high-safety polystyrene is obtained.