A gravitation vortex type water turbine is a new type of water turbine which generates electricity by the gravitational vortex which arises in a tank at the time of drainage. This water turbine is capable of producing energy using low head and low flow rate. Additionally this water turbine has a relatively simple structure. However, since the flow field of this water turbine has a free surface, it is very complicated. It is believed that the flow field around the runner considerably changes with the depth direction. In this study, the performance and flow field around the runner in the position of each blade width direction are investigated by experiments and numerical analyses, and the following facts are clarified. The forward flow region in the water region increases by a tip side. However, the reverse flow region also increases because the air region decreases.
