There is disclosed an optical sensor comprising a holder provided on a circuit board. The holder has first and second cavity means formed therein. The optical sensor further comprises first body means including LEDs combined therewith for directing a light onto an object to illuminate the object with the light and generate a reflected light reflected from the object. The first body means is of a cassette type adapted to be fitted into and held in the first cavity means of the holder for removal. The first body means is electrically connected with the circuit board to complete a first electronic circuit through the LEDs when fitted into the first cavity means. The optical sensor further comprises second body means including photo detecting means combined therewith for receiving and detecting the reflected light reflected from the object. The second body means is of a cassette type adapted to be fitted into and held in the second cavity means of the holder for removal. The second body means is electrically connected with the circuit board to complete a second electronic circuit through the photo detecting means when fitted into the second cavity means.