PURPOSE: To prevent the deterioration of liquid products or their raw material liquid and to simplify the control in a production/distribution process by connecting a deaeration unit and a washing unit through a valve and washing the equipment itself before deaeration treatment.

CONSTITUTION: A deaerator consists of a deaeration unit 10 for removing dissolved gas from liquid products or their raw material liquid, and a washing unit 20 connected to the deaeration unit 10 in parallel and provided with a washing chemical tank 21 and a washing water feed port 22 en route. The deaeration unit 10 and the washing unit 20 are connected through valves V2, V5 which are selectively opened and closed. by washing equipment itself before deaeration treatment, fouling, offensive odors, etc., during a dormant period and smells of liquid products in other product tanks or their raw material liquid are washed and removed by switching operation of the valves to perform deaeration treatment with the deaerator and piping being always kept clean. Therefore, the deterioration of the quality of liquid products due to dissolved oxygen is prevented without other odors and smells being added to the liquid products.