To provide a coupling jig for testing a water-full piping system to inspect a piping system for water leakage such as a water supply system or a drain system, etc., to be laid in a building.

This coupling jig 20 for testing a water-full piping system inspects a piping system 1 for water leakage by charging a pipe coupling 2 with it from a peripheral wall opening part 6 of the pipe coupling 2 of the piping system 1 arranged along the vertical direction. Thereby, in this case, it is furnished with blocking means 27, 28 to seal an upper space in the pipe coupling 2 free to open and close by closely making contact with a wall surface of an upper part passage 14 of the pipe coupling 2 free to make contact with and to separate from it, sealing holding means 21, 23, 29 to hold the blocking means 27, 28 by making them closely contact with the upper part passage 14 of the pipe coupler 2 and change-over means 29, 30, 44, 50 to communicate or block the upper space in the pipe coupling 2 above the blocking means 21, 23, 29 a piping system outer space outside of the peripheral wall opening part 6 of the pipe coupling 2 positioned below the blocking means 27, 28 to and from each other free to open and close.