1. An interpolated frame generating method of generating a new interpolated frame inserted between sequential frames, by using a motion vector obtained by block matching processing to calculate a sum of absolute difference which is a cumulative sum of absolute difference values of pixel values in blocks set in a plurality of input frame images, comprising: a first detecting of detecting at least one potential motion vector by block matching processing between the input frame images, using a first block of a fixed size; a second detecting of detecting a motion vector by block matching processing between the input frame images, using a second block having a fixed size larger than that of the first block; and generating the interpolated frame by using the potential motion vector when the number of the potential motion vector detected by the first detecting is one, and generating the interpolated frame by using a potential motion vector, which is closest to the motion vector detected by the second detecting among the potential motion vectors, when a plurality of motion vectors are detected by the first detecting, wherein, the first detecting includes selecting one motion vector to be used in process when a plurality of potential motion vectors are detected; comparing a sum of absolute difference value of the selected motion vector with sum of absolute difference values of adjacent a plurality of motion vectors; and extracting the selected motion vector as a potential motion vector when the sum of absolute difference value of the selected motion vector is smaller than the sum of absolute difference values of the adjacent a plurality of motion vectors.