To provide a heat retaining material excellent in foamability, thermal weldability and heat resistance.

The heat retaining box as a heat insulating material comprises: a resin comprising a component derived from an ester obtained by a reaction of 10-20C polycyclic terpenol with methacrylic acid and a styrene monomer-derived component; and a volatile foaming agent. (1) The resin comprises the ester-derived component of >1 pts.wt. and <30 pts.wt. based on 100 pts.wt. of the total of the ester-derived component and the styrene monomer-derived component. (2) A foamable polystyrene-based resin particle has a surface layer of 0.35-6.00 absorbance ratio X (D1730/D1600) (wherein D1730 is the absorbance at 1730 cm-1 peak of an infrared absorption spectrum obtained by infrared spectroscopic analysis; D1600 is that at 1600 cm-1 peak thereof). (3) A central part of the absorbance ratio Y (D1730/D1600) smaller than the absorbance ratio X. The heat retaining box comprises an expansion-molded product of the foamable polystyrene-based resin particle.