1. An illuminating device, comprising;a light source that emits excitation light;
a wheel that includes a wheel substrate through which the excitation light is transmitted and an illuminating light generating member that is provided on a first surface of the wheel substrate to generate, at a side of the first surface, effective light as illuminating light by the excitation light;
a photo detector that is arranged on a side of a second surface that is opposite to the first surface of the wheel substrate to detect light that passed through the wheel; and
a control unit that determines whether a defect or a degeneration has occurred in the illuminating light generating member, based on the light detected by the photo detector.