An optical sensor having a common light sensing circuit for synchronously sensing a plurality of color light signals is provided. A plurality of photoelectric components respectively convert the plurality of color light signals into a plurality of photocurrents. A plurality of gain amplifiers respectively multiply the plurality of photocurrents by a plurality of gains to output a plurality of amplified photocurrents. An arithmetic circuit adds up the plurality of amplified photocurrents to output a total amplified photocurrent signal. A common analog-to-digital converter converts the total amplified photocurrent signal into a digital signal. A counter circuit counts bit values of the digital signal to output a counting signal.