To provide a highly reliable apparatus for measuring mercury contained in the gaseous medium, in which the outlet port 3b of the reducing tube and the gas-liquid separator, both susceptible to deposition of stain, can be automatically cleaned to avoid reduction of the sensitivity of the measuring instrument, the apparatus for measuring mercury contained in the gaseous medium includes a reducing tube 3 for reducing divalent mercury contained in a gaseous medium to zerovalent mercury, a gas-liquid separator 4 for removing a liquid component (drain) from the gaseous medium that has passed through the reducing tube 3, a measuring instrument 6 for measuring the quantity of the mercury contained in the gaseous medium delivered from the gas-liquid separator 4, and a cleaning system 10 for running a cleaning liquid from an outlet port of the reducing tube 3 past the gas-liquid separator 4 to emerge outwardly of the gas-liquid separator 4.