In a monitoring apparatus, an optical flow calculator calculates an optical flow for each of selected pixel regions included in at least one pixel region group of an image. The optical flow for each of the selected pixel regions includes information about a direction and amount of movement of a corresponding part of the at least one target object. An adjuster calculates the sum of areas of all the pixel regions included in the at least one pixel region group of the image, and adjusts a selected number of the optical flows to be matched with the at least one pixel region group as a function of the calculated sum of the areas of all the pixel regions included in the at least one pixel region group.