To provide a mercury measuring apparatus of a closed system capable of performing automatically a process ranging from collection to injection of the sample composed mainly of hydrocarbon, which is effective to accomplish a highly reliable measurement. The mercury measuring apparatus 100 includes a column 1 filled with a first adsorbent 11 and a reducing agent 13, an injecting unit 3 for injecting a sample into the column 1, a first heating unit 12, a second heating unit, a mercury collecting tube 18 filled with a second adsorbent 17, a third heating unit 19, a mercury measuring unit 2, gas flow passages FP1, FP2 and FP4, flow switching valves V1 to V3, V5 and V6 for selecting one of the gas flow passages FP1, FP2 and FP4, and a control unit 4.