It is intended to investigate the mechanism of drag reduction of a turbulent flow in a channel by wall oscillation normal to main flow, in this report. DNS is carried out in which one wall of a two-dimensional channel is stationary but the other is in sinusoidal oscillation normal to the mean flow. The most effective oscillation period is adopted and about 35% reduction in drag is confirmed. For this case, generation terms of quasi-streamwise vorticity are examined from various points of view. It is shown that quasi-streamwise vortices which play a key role in sustaining wall turbulence are most strongly affected in the buffer layer. Vorticity production rate fluctuates with the phase of wall oscillation and is enhanced at some definite favorable phase but suppressed as a whole, in one period of oscillation. The stretching effect as well as tilting effect is important. However, the latter affects mainly weak chaotic vortices and hence the role of the former which modulates coherent quasi-streamwise vortices is more substantial. The suppression of vorticity generation by stretching is due to the decrease of inclination angle of a vortex to a wall. It is demonstrated that the fall of production rate of quasi-streamwise vorticity due to stretching effect is substantial for the phenomenon. The periodical fluctuation of the attitude angle, i.e., the inclination angle to the wall and the slant angle of a vortex to mean flow, aggravates the production. This mechanism is a straightforward extension of the one commonly found in the usual near wall turbulence.