An LCD driver compares an image signal with a count of a counter repeatedly counting a clock. Based on the result of the comparison, the LCD driver outputs pulses each having a duty factor in accord with the image signal. When the count is not between first and second predetermined numbers, the frequency of the clock supplied to the counter is switched from a fundamental frequency to a low-frequency, thereby controlling the width of a drive voltage of a liquid crystal display cell and correcting the image signal in association with the S-shape characteristic of optical transmittance of the liquid crystal display cell. This LCD driver is advantageously simple in structure.