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 extracting of comparing a sum of absolute difference corresponding to a motion vector with sums of absolute difference corresponding to motion vectors adjacent to the motion vector, for the individual motion vectors, and extracting a potential motion vector having a sum of absolute difference smaller than any of the sums of absolute difference corresponding to the adjacent motion vectors, as a potential motion vector used in the generating, when a plurality of potential motion vectors are detected.