An imaging device comprising: a pixel unit having a plurality of pixels arranged to form a plurality of rows and a plurality of columns; a readout unit that divides the pixel unit into a plurality of pixel blocks in accordance with a first division pattern, each pixel block including at least two of the plurality of pixels, and combines signals from the at least two of the plurality of pixels included in one pixel block of the plurality of pixel blocks to generate one signal for each of the plurality of pixel blocks; a detection unit that detects a change in signal values between frames by comparing signals that are generated by the readout unit in one frame for the plurality of pixel blocks with signals that are generated by the readout unit in a subsequent frame for these pixel blocks, and by determining that there is a change in the signal values if the signal difference between said frames exceeds a predetermined threshold; and a control unit that, in response to the detection unit detecting a change in the signal values, controls the readout unit so as to output a signal individually from each of the plurality of pixels, wherein the control unit controls, in response to the detection unit not detecting a change in the signal values, the readout unit to divide the pixel unit into a plurality of pixel blocks in accordance with a second division pattern different from the first division pattern; and wherein the number of pixel blocks that form a region in the pixel unit in the first division pattern is smaller than a number of pixel blocks that form the same region in the pixel unit in the second division pattern. a pixel unit having a plurality of pixels arranged to form a plurality of rows and a plurality of columns; a readout unit that divides the pixel unit into a plurality of pixel blocks in accordance with a first division pattern, each pixel block including at least two of the plurality of pixels, and combines signals from the at least two of the plurality of pixels included in one pixel block of the plurality of pixel blocks to generate one signal for each of the plurality of pixel blocks; a detection unit that detects a change in signal values between frames by comparing signals that are generated by the readout unit in one frame for the plurality of pixel blocks with signals that are generated by the readout unit in a subsequent frame for these pixel blocks, and by determining that there is a change in the signal values if the signal difference between said frames exceeds a predetermined threshold; and a control unit that, in response to the detection unit detecting a change in the signal values, controls the readout unit so as to output a signal individually from each of the plurality of pixels, wherein the control unit controls, in response to the detection unit not detecting a change in the signal values, the readout unit to divide the pixel unit into a plurality of pixel blocks in accordance with a second division pattern different from the first division pattern; and wherein the number of pixel blocks that form a region in the pixel unit in the first division pattern is smaller than a number of pixel blocks that form the same region in the pixel unit in the second division pattern.