A state quantity estimating device is configured to: calculate a total enthalpy given to the air by the impeller, an adiabatic compression efficiency of the compressor, and a diffuser outlet flow speed; allocate, to an internal energy and a pressure energy, a value obtained by subtracting a kinetic energy based on the diffuser outlet flow speed from the total enthalpy, based on the adiabatic compression efficiency; and calculate a compression temperature of air flowing into the outlet surface, using an operational equation that includes, as variables, the internal energy, and an inlet temperature of air prior to being compressed by the impeller, and calculate a compression pressure of the air flowing into the outlet surface, using an operational equation that includes, as variables, the pressure energy, and an inlet pressure of the air prior to being compressed by the impeller.