An edge exposing apparatus includes a photo detector and a lighting time measuring circuit for detecting a decrease in performance of light source units. The same lighting time measuring circuit has a function to detect a ready state (i.e. stabilization of luminous intensity) of the light source units capable of performing edge exposure. When the lighting time measuring circuit detects a lighting time having reached a preset value, or when the photo detector detects a luminous intensity below a preset value, the light source units are determined to have lowered in performance, and are switched off. The lighting time measuring circuit measures a lighting time from a startup (switch-on) of the light source units, and when the lighting time reaches a preset value, the light source units are determined to have attained the ready state and edge exposure is started with the light source units. When a decrease in performance is detected, one light source unit is started while continuing edge exposure with the other light source unit. When a ready state is detected, the edge exposure with the light source unit is terminated (switch-off), and edge exposure is performed with the light source unit having attained the ready state.