Abstract: A beam quality control device includes an optical fiber, a stress-applying portion, and a temperature controller. The optical fiber has a core and a cladding that surrounds an outer peripheral surface of the core. The stress-applying portion is in surface-contact with at least a portion of an outer peripheral surface of the optical fiber. The stress-applying portion has a coefficient of thermal expansion of the stress-applying portion that is different from a coefficient of thermal expansion of the cladding. The temperature controller controls a temperature of the stress-applying portion. The stress-applying portion contracts or expands due to the temperature being changed by the temperature controller such that a distribution of external force applied by the stress-applying portion to the cladding becomes non-uniform in a peripheral direction of the cladding.