To provide a decompression type counterflow preventing device capable of being embodied in a small size having such a functioning as to distinguish whether the exhaust water from the device is due to normal functioning or to any failure in the device, whereupon an alarm is issued in the case the exhaust water is due to failure in the device.

A failure sensing device 41 is installed below an exhaust hole 27 provided in the intermediate chamber 13 of a decompression type check valve. The water exhausted from the exhaust hole in the intermediate chamber is received by a water receiving vessel 65, and when the water level lies over a certain level, it is sensed by a water sensor 43. The water is discharged from a discharge port 69, and an alarm is issued in the case water is exhausted continuously from the exhaust hole and the water sensing by the sensor takes place continuously for a certain predetermined period of time.