A method is disclosed for controlling a light source installed in a liquid crystal display device. An embodiment includes a liquid crystal display panel for displaying an image by virtue of having liquid crystals whose orientation changes in response to application of a voltage; a backlight unit with a built-in PWM light modulation type LED for emitting light to be supplied to the liquid crystal display panel; and a control unit for controlling the liquid crystal display panel and the backlight unit. When the response speed of liquid crystal molecules is relatively high, the LED is driven at a relatively low drive frequency, and when the response speed of the liquid crystal molecules is relatively low, the LED is driven at a relatively high drive frequency.