This paper describes an experimental study of mean flow properties in the three-dimensional turbulent boundary layer over a ridge installed on a flat plate with a yaw angle of 30°. The initially two-dimensional turbulent boundary layer on a flat plate is subjected to the three-dimensional perturbation by the yawed ridge, and then the three-dimensional boundary layer relaxes to the two-dimensional one on a flat plate downstream of the ridge. Mean velocities have been measured using a V-type hot-wire prove. The changes in the velocity profiles are investigated by utilizing a polar diagram and logarithmic velocity distributions. Wall shear stress is estimated by the same methods as in the two-dimensional turbulent boundary layer. The extent of the relaxation region, a region where the variations of the boundary layer parameters such as the wall shear stress and integral thicknesses are remarkable, is made clear.