To prevent a foam breaking phenomenon from occurring at the time of foaming and to obtain a container with good appearance and excellent in mechanical strength by using, as a molding material, a polystyrene-based resin whose melt index and melt tension satisfy specific relational formulae.

A polystyrene-based resin whose melt index(MI) at 200°C with a load of 5 kg is 1 to 10 g/10 min wherein MI and melt tension(MT) at 190°C satisfy relational formulae of MT≥-9MI+48 (where 1<MI≤4) and MT≥-MI+16 (where 4<MI≤10) is used as a molding material for a foaming container. The polystyrene containing resin satisfying the characteristics is excellent in balance between flowability and melt tension. therefore use of the polystyrene-based resin allows a container which has good appearance and is excellent in mechanical strength such as impact resistance to be obtained without causing a foam breaking phenomenon at the time of foaming.