Embodiments of a novel control device and associated techniques for controlling a light path length of a resonator to allow resonance in a mode of higher strength are described herein. The control device includes: a drive section that moves at least one reflecting unit in the resonator; and a control section that controls a light path length of the resonator, by causing the drive section to move the at least one reflecting unit so that the laser light that enters into the resonator changes from a state in which the laser light resonates in a first mode of the plurality of modes to a state in which the laser light resonates in a second mode different from the first mode, on the basis of a detection result of a reflected light from the resonator.