1. A pixel signal processing apparatus that, given a group of pixel signals from pixels arrayed on a two-dimensional plane, each pixel having one of a first to an N-th spectral characteristic, generates a pixel signal having an L-th spectral characteristic at a pixel position of interest where there is a pixel signal having a K-th spectral characteristic (K and L being different integers between 1 and N, inclusive), comprising: a selector for selecting one of the arrayed pixels for the pixel position of interest; low-pass filters corresponding to the K-th spectral characteristic and the L-th spectral characteristic, respectively, the low-pass filters processing a plurality of pixel positions in an area neighboring the pixel position of interest; a regression analysis device for performing a regression analysis in a plurality of pixel positions in the area neighboring the pixel position of interest, using the pixel signals or outputs of the low-pass filter corresponding to the K-th spectral characteristic as an explanatory variable and the pixel signals or outputs of the low-pass filter corresponding to the L-th spectral characteristic as a purpose variable, to calculate a regression line, 
 y=a·x+b (‘y’ being the pixel signal or the output of the low-pass filter having the L-th spectral characteristic, ‘x’ being the pixel signal or the output of the low-pass filter having the K-th spectral characteristic, ‘a’ and ‘b’ being constants representing the slope and intercept of the regression line), expressing a correlation of the pixel signals having the K-th spectral characteristic with the pixel signals having the L-th spectral characteristic; and a calculating device for determining the pixel signal having the L-th spectral characteristic at the pixel position of interest by applying a conversion formula based on the regression line to the pixel signal having the K-th spectral characteristic at the pixel position of interest, wherein the selector sequentially selects different ones of the arrayed pixels for the pixel position of interest and, for each selected pixel position of interest, using the regression analysis device and the calculating device to determine the pixel signal having the L-th spectral characteristic, the low-pass filter corresponding to the K-th spectral characteristic is configured to process a particular pixel position by outputting a mean value of pixel signals having the K-th spectral characteristic within a predetermined area of the particular pixel position, and the low-pass filter corresponding to the L-th spectral characteristic is configured to process the particular pixel position by outputting a mean value of the pixel signals having the L-th spectral characteristic within the predetermined area of the particular pixel position.