We present the results of studying the start-up scenario for a high-power pulsed gyrotron operated at the TE 53 mode for two variants of the current-voltage characteristic of a three-electrode magnetron-injection electron gun. In the standard variant of feeding the voltage to the first anode from the resistive divider, sequential generation of the TE 73 and TE 63 modes, and, finally, the operating TE 53 mode was observed at the edge of the pulse. The current-voltage characteristic for which only the operating mode is excited is obtained by decreasing the growth rate of the voltage applied to the first anode.