To provide plaster which is more excellent in waterproofness and strength than conventional one, and to provide a plaster material and a high-Ca polymer solution for producing the plaster, and a plaster application method using them.

A high-Ca polymer solution is prepared by dissolving quick lime and/or slaked lime and a polymer material in an acidic aqueous solution. A plaster material, which is suitable for plaster and enables application of plaster having excellent waterproofness and high strength, is made by adding the high-Ca polymer solution to a plaster material containing quick lime and/or slaked lime and a fine aggregate as major components.