PURPOSE: To constantly maintain the steam amount suitable for the respective foods by controlling a steam-amount control valve and a steam pressure-reducing valve based on a signal from a temperature sensor.

CONSTITUTION: A fluid to be treated is sent through a pump 3 to a plate-type heat exchanger 1 and steam as a heating medium is supplied to an in-line heater 5. Based on a signal from a temperature sensor 8, a steam amount control valve 9 and a steam pressure-reducing valve 10 are controlled. The pressure- reducing valve is controlled through pressure-reducing air valves 12 and 13 for the upper limit and the lower limit.