In a cross-flow turbine, when the opening ratio of the guide vane is narrow, the runner chamber side channel between the nozzle walls and the guide vane becomes an expanding channel, and these cause a separation and a turbulence increasing of the flow under most conditions. As a countermeasure for this problem, we present a method to reduce the pressure drop and the loss, by bringing about separation of the flow. This method is the sucking naturally atmosphere through the slit on the wall by using the negative pressure at the constricted area of the upstream side of the expanding channel. In this report, the performances of a cross-flow turbine, in both case of atmosphere intake from slit of nozzle wall which is pressure method and from hole of runner chamber wall which is conventional method, are investigated by using CFD and experimental test. As a result, an experimental turbine sucking air from the slit on the nozzle wall increases 3.9% of the efficiency compared to that in the case of sucking air from the hole on runner chamber wall, under a guide-vane opening ratio of 30%.