A brightness adjusting device includes a multiplication circuit, which multiplies a vertical signal by a predetermined multiplication ratio to output a multiplication signal, a differential circuit that distinguishes between different multiplication signals, a serrated wave generator that charges/discharges a condenser according to a signal output from the differential circuit and generates a serrated wave having constant amplitude independently of a frequency of the multiplication signal, a brightness adjusting voltage generator that generates a reference voltage to determine a pulse duty, and a comparator that outputs a pulse signal by comparing a voltage of the serrated wave with the brightness adjusting voltage. The brightness adjusting voltage generator includes a circuit that outputs a variable voltage, and the multiplication circuit includes a circuit having a variable multiplication ratio.