Mercury (Hg) is a toxic element of global concern, and the Minamata Convention on Mercury came into force in 2017. Intensive discussions on the reduction of Hg emissions and the environmental monitoring of Hg have been undertaken through the United Nations Environment Programme. To reduce the environmental risks posed by Hg, a simple and rapid detection system for Hg in the environment is needed. We developed a simple Hg detection device based on a quartz crystal microbalance (QCM-Hg). It utilizes the direct reaction of Hg and the Au electrode of the quartz crystal element via the phenomenon of Au–Hg amalgamation. In this study, experiments were conducted to examine the new measurement system and to clarify its detection characteristics. The experimental results show a positive relationship between Hg concentration and frequency shift. When the gas flow rate is high and the measurement time is long, the Au–Hg amalgam reaction proceeds more efficiently and the frequency shift increases. On the basis of the calibration curve obtained, it is possible to determine the Hg gas concentration. In the case of 10 μg Hg/m3, a frequency shift of 4.8 Hz/min is obtained using a 30 MHz quartz crystal element of the surface mount device (SMD). This new detection system enables easy measurement of very low concentrations of Hg. These results revealed that QCM-Hg is a useful Hg measurement system.