To provide a less dangerous, rapid, and safe dissolving method in which an expended polystyrene is safely ground in a relatively short time, and then rapidly dissolved.

This liquid is one to dissolve an expanded polystyrene and comprises a halogen flame retardant org. solvent and an org. peroxide like benzoyl peroxide. This dissolving method of the polystyrene foam comprises a process of coating or injecting an electroconductive soln. or a surfactant aq. soln. having a function of ion conduction or electron conduction to a block of the expanded polystyrene and then grinding the polystyrene, and a process of dissolving the ground expanded polystyrene by a polystyrene foam dissolving liquid comprising a flame retardant org. solvent and an org. peroxide.