PURPOSE: To prevent accidents, etc. to be caused by backflow in two systems of pipings by automatically shutting off both pipings from each other when either of the pressures of the respective pipings or the differential pressure between both pipings deviates from a reference value for judging abnormality.

CONSTITUTION: The pressure of a piping 1 on the side A, more particularly the pressure on the upper stream side than the branching part 5 of a connecting pipe 4 is maintained at pressure higher by prescribed differential pressure than the pressure of a piping 2 on the side B at all times, between a piping system of fluid A and a piping system of fluid B. At the same time the pressures of both pipings 1, 2 are controlled respectively to prescribed pressures so that the state in which backflow is difficult to arise is maintained normally by said pressures. On the other hand, a check valve mechanism 19 is provided to the pipe 4, and if the mechanism 19 is operated in accordance with the detection signal thereof when the pressure of the piping 1 on the side A or the pressure of the piping 2 on the side B or the differential pressure between both pipings 1 and 2 indicates an abnormal value, the flow between A and B at the pipe 4 is shut off and isolated.