A beam expander includes a first lens unit including one of an SLM or a VFL, a second lens unit being optically coupled to the first lens unit and including one of an SLM or a VFL, and a control unit controlling focal lengths of the first and second lens units. A distance between the first and second lens units is invariable. The control unit controls the focal lengths of the first and second lens units such that a light diameter D1 of light input to the first lens unit and a light diameter D2 of light output from the second lens unit are different from each other.