1. A motion detection apparatus, comprising: a first detector for detecting a motion of a partial object scene captured in each of a plurality of blocks allocated to an object scene based on image data repeatedly outputted from an imager; a reducer for reducing a weighted amount allocated to a block in which a partial object scene having a contrast below a reference is captured, out of a plurality of weighted amounts respectively allocated to the plurality of blocks; a second detector for detecting a motion of the entire object scene by performing a weighted average process in which the plurality of weighted amounts are referenced on the motion detected by said first detector; a fist obtainer for repeatedly obtaining a minimum correlation value of a luminance difference between screens of the image data outputted from said imager, corresponding to each of the plurality of blocks; and an increaser for executing a process for increasing the reference according to an increase of the minimum correlation value obtained by said first obtainer, corresponding to the block in which the partial object scene having the contrast below the reference is captured.