A backup power supply control system includes: a first semiconductor switch that switches a main power supply line to an electrically conductive state or an electrically non-conductive state; and an auxiliary power supply switch that switches an auxiliary power supply line to the electrically conductive state or the electrically non-conductive state. A first driving unit controls, in a non-failure state, a second semiconductor switch, connected between a gate and source of a first semiconductor switch, OFF and thereby controls the first semiconductor switch ON. The first driving unit ensures, in the failure state, at least a gate-plateau voltage of the first semiconductor switch as a drive voltage for the second semiconductor switch. When a failure detection unit detects the failure state, the first driving unit controls the first semiconductor switch OFF by controlling the second semiconductor switch ON and a second driving unit controls the auxiliary power supply switch ON.