PROBLEM TO BE SOLVED: To provide a light source device making best use of a light source without restricting a quantity of light more than necessary while securing safety of a subject.SOLUTION: When a geared motor 141 rotates, an optical sensor 142 moves to an optical axis to face a main light source 168 and the optical sensor 142 measures the quantity of light. A system control circuit 111 reads quantity of light threshold from a scope memory 201 and determines whether or not the quantity of light of the main light source 168 is larger than the quantity of light threshold. When the quantity of light of the main light source 168 is larger than the quantity of light threshold, current to be flown to the main light source 168 is decreased until the quantity of light of the main light source 168 is equal to or smaller than the quantity of light threshold.
