An image acquisition device includes: an image acquisition unit that acquires images of a subject in time series; a hierarchical motion-detection unit that can detect a motion vector between the images acquired by the image acquisition unit, by using images at two or more levels having different resolutions; a pixel-value-change detecting unit that divides the images acquired by the image acquisition unit into a plurality of partial regions and that detects a temporal change of the pixel value at a detection pixel set in each of the partial regions; and a hierarchical-motion-detection setting unit that sets a larger number of levels used in the hierarchical motion-detection unit, for any of the partial regions that includes the detection pixel at which the temporal change detected by the pixel-value-change detecting unit is larger.