This invention relates to a cock for water purifier which has a cock body formed with two compartments partitioned with each other. One of the two compartment is provided with a tap-water passage, and then a water input passage connecting with the water inlet of the water purifier is provided adjacent to the tap-water passage, and a conversion switch for opening and closing the water input and the water output is also disposed in the same compartment. Then, a water output passage connecting with the water outlet of the water purifier is provided in the other compartment of the cock body. In this way, the cock is activated to switch between opening state and closing state of the piping associated with the flow-in of the tap water to the water purifier or the flow-out of the cleaned water from the water purifier. Therefore, such damages as rupture or water leakage of the water purifier caused by the continuous rendering of impact came from the water-hammer action formed by the sustaining applied pressure of the tap-water to the water purifier during cease of the drinking of cleaned water can be avoided.