To enable washing a closed tank in a short time and effectively by connecting an automatic piping connecting device by which a washer and piping attached to the closed tank are connected to the washer.

A moving closed tank 1 is mounted on a moving tank truck with which an automatic carrier is loaded. After the work is finished, the automatic carrier is moved to a washing station, and the moving tank truck is transshipped to a frame of the washing station from the automatic carrier. And washers 10 and 10' arranged in the washing station and automatic piping connecting devices 30 and 30' are connected, and by the automatic piping connecting devices 30 and 30', the washers 10 and 10' and piping 36 and 36' attached to the tank 1 are connected. After that, the inside of the tank 1 is subjected to gas replacement, and then, washing is performed. In this way, the tank 1 can be washed in a short time and effectively.