PURPOSE: To provide an automatic pressure reducting device which can safely reduce excessive water pressure generated in a pipe line in a pipe line network.

CONSTITUTION: A connecting pipe line 2 is arranged in the middle of a pipe line, and one end is connected to the upstream side of the pipe line, and the other end is connected to the downstream side of the pipe line, respectively. A valve 3 to open and close a passage of the connecting pipe line 2 is arranged in the connecting pipe line 2. An automatic pipe waterway pressure reducing device 1 is provided with a water pressure detector 4 to detect water pressure downstream from the valve 3 inside the connecting pipe line 2 and a valve opening-closing mechanism 5 to control opening of the valve 3 so that the opening of the valve 3 becomes small when the water pressure detected by the water pressure detector 4 is high, and that the opening of the valve 3 becomes large when the water pressure detected by the water pressure detector 4 is low.