A near field optical recording/reproducing apparatus includes a light source, an objective lens to form a spot by focusing light emitted from the light source, a solid immersion lens to generate an evanescent wave by using the spot formed by the objective lens, a photodetector to measure an amplitude of a gap error signal that is totally reflected by the solid immersion lens, and an operating unit to generate a normalized gap error signal by using an amplitude of a driving signal applied to the light source and the amplitude of the gap error signal measured by the photodetector.