We discussed the development mechanism of the streamer channel leading to breakdown in a short air gap, by using an ultra-high speed measuring system. We clarified that the initiation of the secondary streamer leading to breakdown is attributed to an electron injection from the grounded plane electrode to the residual channel made by the primary streamer. In addition, by selective measurement of the discharge current waveform of single streamer channel leading to breakdown, the breakdown was verified to be dominated by the development of the secondary streamer and the subsequent Joule heating process focusing on the single streamer channel.