We developed a high-speed line-scanning camera (HSLSC) for measuring the radial variation of optical intensity of lightning channels. With this system we succeeded in obtaining a few examples of data for natural lightning. From these data we found that the luminous region of return stroke channels may expand with a velocity of more than 105 m/s during the initial stage and then reach the maximum diameter of several tens of meters after about 100 μs from the initiation of the return stroke and finally decrease with time. This suggests the occurrence of lateral discharges along the lightning channel during the propagation of return strokes. ¿ 1998 American Geophysical Union