A wet gas purification method is provided for removing ammonia from a heavy oil gasification gas, such as coal. The method principally involves a washing step during which absorbent is charged into the gas to absorb ammonia and an ammonia treating step wherein absorbent discharged from the washing step is separated into an effluent and an off-gas containing ammonia. The amount of absorbent charged during the washing step is controlled such that the ammonia concentration of gas exiting the washing step is 10 ppm or less.