A DC-DC converter for boosting an input voltage to a higher output voltage includes an inductor, a field effect transistor, a first ON signal generating circuit, a second ON signal generating circuit, an OFF signal generating circuit, a transistor driving signal generating circuit, and a switching frequency varying circuit. The switching frequency varying circuit changes a first ON time with the passage of time, so that the energy of switching noise is diffused without concentrating at specific frequencies. Thus, magnitude of the energy of switching noise in each switching frequency can be reduced.