To improve the stability of spray application of a monolithic refractory in which a water injector is disposed in a material carrier pipe extending from a material supply device to a distal spray nozzle, and application water is injected from the water injector into a spray material that is being carried through the material carrier pipe, a ratio of a flow volume of an application water carrier gas for carrying the application water to be introduced into the water injector to a flow volume of a spray material carrier gas for carrying the spray material is set to 0.07 to 2, and a compressibility index of the spray material is set to 32% or less. Alternatively, a ratio of a flow volume of an application water carrier gas for carrying the application water to be introduced into the water injector to an application water volume is set to 100 to 1,000.