The Minamata Convention on Mercury was adopted at the Conference of Plenipotentiaries held in Kumamoto Prefecture, and in 2017, the treaty came into effect. Mercury (Hg) is a toxic element of global concern. To reduce the environmental risks from Hg, a simple and rapid detection system for Hg in the environment is needed. We developed a simple and highly sensitive detecting method for the detection of Hg gas using a quartz crystal microbalance (QCM) This detection system is utilizing the direct reaction of Hg and Au electrode of the QCM by the phenomenon of Au-Hg amalgamation. This system consists of small loggers equipped with temperature, humidity, pressure and QCM sensors. It was developed for the personal exposure monitoring. When the amount of gas flow rate is large, the Au-Hg amalgam reaction proceeds better and the frequency shift increases. Using a calibration curve, it is possible to determine the Hg gas concentration. This new detection system allows easy measurement of very low concentrations of Hg.