The detailed three-dimensional structure of the flame front disturbance induced by a weak pressure wave has been studied experimentally using a small combustion chamber. The flame front disturbance was recorded by a newly developed high-speed Schiieren system, which gives a view from the direction normal to the flame front. The results were compared with those induced by preferential diffusion mechanisms. The image of a flame front disturbance induced by acceleration caused by weak pressure waves was found to be circular when it was observed from the direction normal to the flame front. This image was much different from that induced by the preferential diffusion mechanisms, which was observed as a net composed of thick lines. These images of the flame front disturbances were compared with those observed in our previous studies, in which the disturbances were observed from the direction normal to the flame propagating direction. Consequently, the flame front disturbances induced by acceleration were inferred to be horn-shaped, while those induced by the preferential diffusion mechanisms were inferred to be ridge-shaped. It is postulated that this difference of the flame front shapes should be attributed to the difference of the mechanisms which, sustain or enhance the disturbances.