An endoscope system includes an imaging unit having an image sensor to image a subject, a light source emitting illumination light for illuminating the subject, and a laser treatment instrument which can emit laser light for treating the subject during emission of the illumination light. A control device for the endoscope system includes a determination unit which determines a state of use of the laser treatment instrument, and a brightness adjustment circuit which adjusts at least one of exposure time in imaging by the imaging unit and the amount of emitted light of the light source based on the result of determination by the determination unit.