1. A depth estimation method comprising the operations of: outputting an optical signal to an object; generating a plurality of frame signals based on a reflected optical signal received from the object and a plurality of gate signals periodically applied to a depth pixel with predetermined phase differences from the optical signal, and providing the plurality of frame signals and frame information of each of the plurality of frame signals for estimation of a depth to the object; grouping the plurality of frame signals into a plurality of frame signal groups which are used to estimate the depth to the object without a depth estimation error caused by an omission of a frame signal, wherein the grouping of the plurality of frame signals is based on whether an omitted frame signal exists in the plurality of frame signals and based on a continuous pattern of the plurality of frame signals; and estimating the depth to the object using each of the plurality of frame signal groups.