A method of adjusting an output bitrate of an image by contrast adjustment includes: determining a maximum subtraction rate of a contrast of the image in response to a brightness of the image; comparing a current output bitrate of the image with a target bitrate while reducing the contrast of the image up to the determined maximum subtraction rate of the contrast of the image; and determining the contrast of the image to be the reduced contrast in response to determining that the reduced contrast of the image corresponds to the target bitrate.