PURPOSE: To prevent contaminating substance from attaching to a vacuum sewage pipe or device, thereby preventing corrosion of a part to which the substance attaches, by forcing washing agent into a sewage pit at every predetermined number of openings and closings of a vacuum valve which effects drawing and transferring of sewage.

CONSTITUTION: Opening and closing of a vacuum valve 4 is detected by a sensor 14 and the detected signal is sent to a signal receiving component 15a, while the number of openings and closings is counted at an opening/closing count detection component 15b. Next, at a comparing and deciding component 15c the number of openings and closings from the component 15b is compared with a preset number of openings and closings. When the number of openings/closings of the valve 4 reaches the preset number, an open/close output is sent apon command from the component 15c to a solenoid valve 13 from an open/close output component 15d to open or close the valve 13, forcing thereby a predetermined amount of washing agent into a sewage pit 2 from a storage tank 11 through a feed pipe 12, where the agent is mixed into the sewage. As a result, contamination and corrosion of a vacuum sewage system can be prevented.