Experimental study was conducted to clarify three-dimensional structure of turbulent shear flow and secondary currents. A fixed wavy bed was used to reproduce the flow pattern over sand ribbons. Cross-sectional distributions of main flow velocity, Reynolds stress and coefficients of eddy viscosity were examined with taking account of the influence of secondary currents. Generation mechanism of secondary currents was investigated on the basis of streamwise mean vorticity equation. From the result, it was confirmed that Reynolds stress (-v′w′) is the driving force for secondary currents and turbulent intensity difference (v′2-w′2) is the resisting force for them, where v′ and w′ are vertical and spanwise velocity fluctuations respectively, and overbar denotes the time average.