Drift assessment of pressure gauges is necessary for longterm pressure observation. We equipped a test system with a deadweight tester and two isothermo baths for month- or year-long sensor evaluation. Here we report the overview of the system, the summary of test run results, and the results of some month- or week-long test of pressure gauges. Detection of kPa or hPa pressure variation or mK temperature variation for years is a key for evaluation of hydrologic or elastic properties in seismogenic zones and for detection of interseismic activities. Subtidal signal is estimated 1 kPa or less. An observatory should detect some kPa change with 3 or 4 digits to evaluate transient events. Our test system was designed for longterm stability assessment of the pressure gauges to be installed in IODP boreholes and on the seafloor. For the case of NanTroSEIZE, C0002 borehole will be extended to the depth of about 7000 m below sea floor and the observatory will be installed there. The pressure and temperature range of the test system was adjusted to meet this purpose. The test system successfully supplied stable reference pressure enough to determine one to two kPa pressure gauge drift for a month. This accuracy was made possible by the automated deadweight tester and 100 mK level stabilized temperature in the isothermo baths. Our result cleary indicates the 4 kPa drift of gauges, which consists of initial 3 kPa change and subsequent 1 kPa linear trend. This drift is comparable to the phenomena which we aim to observe. The capacity of the test system is 13 depthmeters maximum for a test. DO-NET, distributing tens of seismometers and pressure gauges in Kumano basin for real time observation, is another target of this test system. The capacity of the system meets the need of this project.