1. A motion detecting apparatus comprising: a generating unit configured to generate, from an original image being a frame image of a moving image, an image which has the number of gradations smaller than the number of gradations of the original image, in unit of block; a first motion detecting unit configured to perform motion detection based on the image generated by the generating unit and having the number of gradations smaller than the number of gradations of the original image; a calculating unit configured to calculate a parameter indicating a feature quantity of the original image; and a selecting unit configured to select, based on the parameter calculated by the calculating unit, a detection result of the first motion detecting unit or a reference vector as a motion vector.