To explore the effect of acoustic excitation on jet diffusion flames, the authors have investigated the behavior and structure of a methane jet inside a flame. The jet is ejected vertically upward into the air through a tube and burned under the condition of natural convection. It meanders and diverges into two branches under the influence of sound waves propagating transversely across it. Detailed movement of the jet is examined with a high-speed video camera by means of shadowgraphy. It is shown clearly that the meandering behavior is synchronous with the sound wave: its periodicity corresponds to the sound frequency. In order to explore the three-dimensional structure, shadowgraph images for different view-angles are also acquired at the same phase of sound wave. The image viewing in the direction of sound propagation shows that the jet has a symmetric structure. From those images, the three-dimensional structure of the jet is deduced.