1. A motion detection apparatus, comprising: a first detector which detects a motion of a partial scene captured in each of a plurality of blocks allocated to a scene based on image data repeatedly outputted from an imager; a reducer which reduces a weighted amount allocated to a block in which a partial 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 which detects a motion of the entire scene by performing a weighted average process in which the plurality of weighted amounts adjusted by a process of said reducer are referenced on the motion detected by said first detector; a first obtainer which repeatedly obtains a minimum 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 which executes a process for increasing the reference according to an increase of the minimum value obtained by said first obtainer, corresponding to the block in which the partial scene having the contrast below the reference is captured.