The present invention provides a mercury removal system and method for effectively removing a mercury component, which is present in a gas stream in an extremely small amount in wet gas cleaning used for coal or heavy oil gasification, petroleum refining and the like. The mercury removal system in wet gas cleaning comprises a water washing tower for introducing therein a target gas containing a mercury component and transferring the mercury component into an absorbing solution, a flush drum (10) for flushing the absorbing solution discharged from the water washing tower to separate the absorbing solution into a gas component and waste water, an oxidation treatment means (1) for adding an oxidizing agent to the absorbing solution at the preceding stage of the flush drum, and a waste water treatment means for subjecting to coagulation sedimentation treatment the separated waste water containing the mercury component at the following stage of the flush drum to dispose of the mercury component as sludge. The present invention also provides a mercury removal method using the above-described system.