To provide gypsum slurry having stabilized foam which can enhance and control stability of the foam under conditions of using hard water.

The gypsum slurry with stabilized foam is formed by adding foam to a mixture of calcined gypsum and hard water containing at least 300 mg/L of calcium. The foam comprises hard water, dispersed air and surfactants. The surfactants include alkyl ethoxy sulfates, which have a hydrophilic portion comprising an ethoxy group and a hydrophobic portion comprising at least C8, C10, C12 and C14 alkyl chains.