To provide a modified asphalt composition usable as a pressure-sensitive adhesive layer of a water-proof sheet in a method for water-proof construction and having excellent pressure-sensitive adhesiveness to a substrate while maintaining the shear strength in a wide temperature region from normal temperature to low temperatures and to provide a self-pressure-sensitive adhesive water-proof sheet having excellent adhesiveness to the substrate using the modified asphalt composition having the good self-pressure-sensitive adhesiveness.

The modified asphalt composition comprises asphalt, a styrenic synthetic rubber and a pressure-sensitive adhesion imparting agent. Furthermore, the pressure-sensitive adhesion imparting agent contains at least one kind selected from process oil and polybutene. The amount of the pressure-sensitive adhesion imparting agent is 0.5-12 pts.mass based on 100 pts.mass of the total mass of the asphalt and the styrenic synthetic rubber.

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