Magnetic field observations of a planetary magnetosphere and interplanetary space are fundamental in clarifying the structure of the planetary magnetosphere and its interaction with solar wind. The inherent magnetic dipole of Mercury was discovered by flyby observations performed by Mariner 10; however, the structure of the magnetic field and its interaction with solar wind have not been sufficiently investigated. A Japanese fluxgate magnetometer is to be installed on the BepiColombo Mercury magnetospheric orbiter. During the interplanetary space flight and in orbit around Mercury, the sensor of the magnetometer, which is placed on an extension mast, will be exposed to temperatures ranging from −100 to +175 °C. However, the sensor characteristics—sensitivity, magnetic offset and noise—are temperature dependent. In particular, data pertaining to the sensor characteristics over a wide temperature range from −160 to +200 °C are not available. We developed a wide-range temperature (from −160 to +200 °C) testing system with a magnetic shield box and tested two types of sensor core supports (Inconel and ceramic core bobbins). As a result, we obtained the characteristics of the sensor sensitivity, offset and noise and found that the noise performance of the ceramic core bobbin was superior to the Inconel core bobbin.