An apparatus for estimating a level of a substance in blood in a noninvasive manner is provided. The apparatus includes a light source configured to emit light to skin of a user； a plurality of detectors which are disposed at different distances from the light source and configured to detect light signals from the light returning from the skin； and a processor configured to determine a similarity between at least two light signals among the detected light signals and estimate the level of the substance in blood based on the similarity.
