[en] In the joint project for high-intensity proton accelerators, which is promoted by Japan Atomic Energy Research Institute (JAERI) and High Energy Accelerator Research Organization (KEK), a pulsed proton beam of a maximum power of 5 MW from a high-intensity proton accelerator will be injected into a mercury target in order to produce high-intensity neutrons. The neutrons produced by the facility will be utilized in advanced fields of science such as the life sciences, material sciences etc. For the mercury circulation pump installed in the mercury target system, a small-sized, high-reliability pump with superior circulation performance is required. For the mercury circulation pump, it is categorized into two types of pumps. One is an electro-magnetic pump, using an electromagnetic induction, the other is a mechanical pump such as a centrifugal pump, gear pump, etc. From the viewpoint of small-sized pumps, the mechanical pump has an advantage, particularly, a gear pump has the characteristic of high pumping pressure and stable flow with low pump speed compared with a centrifugal pump. But there were not enough data for the mercury gear pump design and its life estimation, because there were no experiences for testing the mercury gear pump all over the world. Therefore, in this study, a gear pump with the capacity to circulate mercury up to 15 L/min was fabricated and tested to understand the gear pump characteristic and to verify its design, under mercury flowing conditions using an existing mercury loop. As a result, the maximum flow rate of 23.1 L/min was achieved at rated pump speed of 350 rpm. The design flow rate of the pump was 15 L/min, therefore, the pump has an enough capacity against the design flow rate and this result verified the validity of the pump design. It was also made clear that mercury flow rate can be evaluated using the pump speed by calibrating a relationship between pump speed and flow rate, because the mercury flow rate was proportional to the pump speed. (author)