Optical observation of 3D streamer image in resistive plate counter

3D streamer image in a resistive plate counter (RPC) were taken by using a glass RPC, an image intensifier and a CCD camera. Observations were simultaneously made for self-discharge in top and side view, operated in the streamer mode with various gas mixtures(Ar/butane/HFC134a). The analysis of the streamer image might be useful for us to design the pickup strip width for the readout. We report on a shape of the streamer in the top and side view as a function of gas mixing ratio.