To provide a lighting system capable of properly determining the timing of replacing a light source.

A memory 25 memorizes luminous flux maintenance characteristics (data showing a luminous flux maintenance curve) corresponding to the lighted state of each LED module 30. A luminous flux maintenance factor computing part 26 computes the luminous flux maintenance factor of the LED module 30 every time the lighted state of the LED module 30 changes under control of a microcomputer 22 for control, based on the luminous flux maintenance characteristics corresponding to the lighted state after the change and a lighting time in the lighted state. That is, the luminous flux maintenance factor is computed using the luminous flux maintenance characteristics different according to the lighted state of the LED module 30.