An object is to provide an optical element that is thin and can gather and reflect light in a specific wavelength range in a predetermined direction, and a sensor including the above-described optical element. The optical element includes a cholesteric liquid crystal layer, in which the cholesteric liquid crystal layer has a liquid crystal alignment pattern in which a direction of an optical axis derived from a liquid crystal compound changes while continuously rotating in at least one in-plane direction, and in a case where a length over which the direction of the optical axis rotates by 180° in the in-plane direction in which the direction of the optical axis changes while continuously rotating is set as a single period, the cholesteric liquid crystal layer has in-plane regions having different lengths of the single periods in the liquid crystal alignment pattern.