PURPOSE: To decrease CWN viscosity without significant reduction in the boiler efficiency, by automatically controlling the amount of water vapor injection based on the calculated target CWM temperature at the water vapor injection so as to realize the target temperature.

CONSTITUTION: The temperature of CWN when its viscosity is reduced to the target value by water vapor injection is calculated, using an operational equipment 32, based on both viscosity-temperature and viscosity-concentration relationships, and transmitted to the controller 33 as the setting value. Thence, based on the comparison of this setting value with the measurement of the CWM temperature at the water vapor injection, a water vapor flow control signal is transmitted from the controller 33 to the control valve 30, thus regulating the amount of water vapor to be injected through said valve 30.