Fully-developed flow in a channel subjected to the spanwise rotation was investigated by means of the direct numerical simulation (DNS). Two steps of rotation number, Roτ=1 and Roτ=2.5, and stationary condition were mainly calculated. The turbulence was enhanced in the pressure side and attenuated in the suction side by increasing the rotation number, but the change was not monotonic.Notable feature was found for the case of Roτ≈ 1 : the flow rate takes its maximum ; it fluctuated in time ; the shift of Reynolds shear stress almost teminated. For higher rotation rate, roll-cell vortices and, as a consequence, the constant-gradient in the mean velocity profile were observed. Furthermore, the log profile was evident only in the region blown from the pressure side by the roll-cell vortices. Due to these observations, it was found that the rotation number 'unity' was a turning point of the change of dominant structures : the near-wall vortices in lower rotation and the large-scale roll-cell vortices in higher.