A water hammer using a drive rod is provided. The water hammer includes a tubular main body having a hollow portion; a socket coupled to an upper end of the main body and having a water pressure supply passage; a cylindrical piston housing connected to the main body; a piston slidably installed in the piston housing, for striking a bit of a bit unit installed at a lower portion of the main body, having a hollow portion through which water is discharged, an annular pressurizing portion protruding on its outer circumferential surface, and a first communication hole connected to the hollow portion; a sliding member fitted into the main body to be coupled to the piston housing, defining a valve installation space, and creating a space portion in which the piston is received when the piston is elevated; a valve member defining the valve installation space into first and third space portions along the length of the piston, the first and second space portions having different cross-sectional areas from each other, and valve member forming a second space portion between the first and third space portions, connected to the hollow portion of the piston and connected to the first space portion when the piston is elevated; and a water pressure supply unit for supplying high pressure water delivered to the water pressure supply passage of the socket to the first and second space portions.