A Darrieus-type open hydro turbine with inlet nozzle has been proposed for utilization of extra-low head hydropower though a ducted type is in general used, consisting of an intake, runner section and draft tube. The open turbine means the simplified one, which has no runner casing and no draft tube. By installing inlet nozzle for the open turbine, the turbine efficiency is never deteriorated in comparison with the ducted one. The role of inlet nozzle is to increase generated torque with higher efficiency on the upstream path of the Darrieus blade passing and to decrease the torque with lower efficiency on the other path by removing the runner casing as the Darrieus turbine is cross flow type. In the present paper, the more simplified open turbine, consisting of only the inlet nozzle and the quarter covered upper casing to support the runner shaft, is experimentally examined. It is found that the depth of water level, covering or uncovering the runner in downstream pond, which might be changed by flow rate in practical use due to river-flow type turbine, influences on the turbine performance. When the depth of water level is kept deeper than the runner height of vertical axis turbine, the performance is almost the same as that of the case with whole covered upper casing. With decreasing the water level shallower than the runner height, the generated power and the efficiency become deteriorated due to making the wavy motion of the water surface at first. Then the stable operation at the maximum efficiency point in normal condition cannot be obtained. The cause on the mechanism of performance deterioration is considered with visualization of fluid behavior around Darrieus blades.