In a conventional display device using light emitting elements, white color cannot be accurately displayed since the luminance of each sub-pixel varies for each color. Therefore, in order to achieve the uniformity in luminance of each sub-pixel, the amount of current or the current density flowing in light emitting elements is required to be changed for each color. In that case, a power source is necessarily provided for each color, thus an external circuit is increased, leading to increase in the cost and power consumption. According to the invention, a variable channel-size MOS transistor is used as a transistor for driving a light emitting element. When a signal voltage applied to a control gate is controlled for each sub-pixel, the amount of current or the current density flowing in light emitting elements can be adjusted and luminance of each sub-pixel can also be adjusted for each color.