A brightness adjusting device has: a warm-up time determining unit which calculates a temperature difference between the outside temperature detected by an outside temperature detecting unit and the inside temperature detected by an inside temperature detecting unit and reading and determining a warm-up time corresponding to the calculated temperature difference, as an estimated warm-up time, from a product inside temperature table stored in a product inside temperature table storing unit; a target brightness value determining unit which reads and determines a target brightness value, which corresponds to the estimated warm-up time read and determined by a warm-up time estimating unit, from a target brightness value table stored in a target brightness value table storing unit; and a brightness value setting unit which sets a brightness value of the display device according to the target brightness value read and determined by the target brightness value calculating unit.