1. An image processing apparatus, comprising: a processor; and a memory device storing instructions which when executed by the processor, cause the processor to: (a) set a plurality of reference blocks having a size equal to that of a target block which is formed with a predetermined size in a target screen image, the plurality of reference blocks being formed from a plurality of pixels within a search range that is set in a reference screen image, the reference screen image being different from the target screen image, the reference screen image including a first number pixels, the search range being set to include a second number of pixels, the second number of pixels being less than the first number of pixels; (b) for each of the reference blocks, determine a correlation value between the target block and each of the reference blocks; (c) determine: (i) the highest value from among the determined correlation values; and (ii) a maximum value from among the correlation values except the highest value; (d) detect a motion vector of the target block as a displacement amount of the reference block with regard to which the highest value of the correlation value is calculated from the target block; (e) calculate the difference between the highest value of the correlation value and the maximum value of the correlation values except the highest value; and (f) create an index to reliability of the detected motion vector based on the calculated difference between the highest value of the correlation value and the maximum value of the correlation values except the highest value.