To provide a modified asphalt composition forming a hardened film having suppressed tackiness and excellent in adhesiveness and tensile strength, the hardened film having suppressed interlayer blister in lamination, suppressed tackiness and excellent tensile strength, and a waterproofing method by which a waterproof layer having excellent working efficiency, water proofing property, strength and elasticity is formed.

The modified asphalt composition contains a polymer particle, an emulsifier, asphalt, a silane coupling agent (the silane coupling agent contains amino group and alkoxy group) and an epoxy resin, wherein the polymer particle contains a particle comprising a conjugated diene-based polymer, the content of the silane coupling agent is 0.2-18 mass% and the content of the epoxy resin is 0.4-23 pts.mass.

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