1. An imaging device comprising: an imaging signal generation unit configured to capture images and generate an imaging signal indicating a pixel value for each pixel in a plurality of pixels constituting a sequence of temporally consecutive frames; an intra-plane pixel summation unit configured to: receive the imaging signal generated by the imaging signal generation unit, sequentially specify the pixels in each of the consecutive frames, select, for each specified pixel, an area consisting of pixels with high mutual correlation, from among a plurality of areas having predetermined relative positions or orientations with respect to the specified pixel, sum the pixel values of the pixels in the selected area, output a resulting sum as an intra-plane sensitized signal of the specified pixel, and output a pixel summing pattern code indicating the relative position or orientation of the selected area; and an inter-plane pixel summation unit configured to: sequentially specify the consecutive frames as a frame of interest, sequentially specify the pixels in the frame of interest as a pixel of interest, select, for each pixel of interest, a pixel from each of one or more frames neighboring the frame of interest, on a basis of correlations of the intra-plane sensitized signal of the pixel of interest with the intra-plane sensitized signals of a pixel positioned identically to the pixel of interest and pixels in a neighborhood of the identically positioned pixel, results of comparisons of the correlations with a correlation decision threshold value, and the relative position or orientation of each of the selected areas with respect to the specified pixel, add the intra-plane sensitized signals of the selected pixels in the one or more frames neighboring the frame of interest to the intra-plane sensitized signal of the pixel of interest, and output a resulting sum as a three-dimensionally sensitized signal.