Flow patterns were visualized by the LIF method in order to elucidate the mechanism of advective mixing in a non-element mixer. Branch flows were injected periodically into the main flow through perpendicular branch pipes. The effects of the number of branch flows and the period of injection on the stretching and folding were investigated experimentally. The boundary length between two fluids L was evaluated in the cross section parallel to the main flow. It was observed that branch flow injection caused a backward flow immediately after the injection and it suppressed the increase in L, but after that L increased due to the increase in the folding. It was also found that the time variation in L depended on the period of injection. Though injected fluid flowed more slowly as the number of branches increased, L increased more due to the remarkable effect of stretching and folding.