An illuminating device includes: a light source emitting excitation light; a wheel including a wheel substrate transmitting the excitation light and an illuminating light generating member provided on the first surface of the wheel substrate to generate illuminating light by the excitation light; a photo detector, arranged on the second surface side that is opposite to the first surface of the wheel substrate, detects light that passed through the wheel; and a control unit that determines whether a defect or degeneration has occurred in the illuminating light generating member, based on the light detected by the photo detector.