Gravitation vortex-type water turbines use gravitational vortex generated when water is guided into the tank and drained from the hole at the bottom of the tank. When a circular tank is used, a runner inlet flow becomes uneven circumferentially, resulting in poor performance. To control the flow in the tank and make it uniform in the circumferential direction, we used a volute tank to elucidate the impact of tank geometry on the performance and flow field of the gravitation vortex-type water turbines. We further investigated the performance and flow field of the water turbine using volute and circular tanks through experiments and free surface flow analysis. We compare the performance and flow field of the volute tank to those of the circular tank. We found that the effective head and turbine output of the volute tank decreased more than those of the circular tank, whereas the water turbine efficiency of the volute tank improved more than that of the circular tank at low to medium rotational speeds. This is because the theoretical head of the volute tank was smaller than that of the circular tank, but the runner inlet flow was uniform in the circumferential direction, and the loss in the tank, which is the dominant loss in the circular tank, was greatly reduced.