Abstract: A mixing system including: a backup heater which increases an internal pressure of a pressurizer; a spray valve which decreases the internal pressure of the pressurizer; a pressure sensor which detects the pressure inside the pressurizer; and a pressure control unit which performs a feedback control on a spray valve so that the pressure becomes a target pressure based on a detection pressure value detected by the pressure sensor, wherein the pressure control unit includes a PID control unit which performs a feedback control when heating the coolant by the backup heater and outputs a pressure controller signal and a bias setting unit which sets a bias toward the operation side of the spray valve with respect to the pressure controller signal, and wherein the spray valve is operated based on the pressure controller signal subjected to the setting of the bias.