To provide an adhesive composition for expanded polystyrene, capable of adhering a waterproof sheet on a heat insulating material to rigidly fix when performing athermalizing and waterproofing execution of adhering rubber waterproofing sheet on a basis through the heat insulating material.

The adhesive composition for expanded polystyrene, usable for adhering a waterproofing sheet 7 on a heat insulating material 2 in an athermalizing and waterproofing method adhering the waterproofing sheet 7 on a basis 1 through the heat insulating material 2 is provided. The adhesive composition for expanded polystyrene is prepared by compounding 10-80 pts.mass tackifier and 300-800 pts.mass solvent comprising at least aliphatic hydrocarbon solvent, per 100 pts.mass elastomer. The aliphatic hydrocarbon-based solvent is low in polystyrene solubility, damages no heat insulating material of polystyrene, and can adhere to the rubber waterproofing sheet 7 by coating the adhesive composition on the heat insulating material.

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