1. A signal processing apparatus comprising: a pixel array where a plurality of pixels for storing data values are arranged; and a noise removing unit suitable for reflecting a data value of an adjacent pixel and a gain value on a data value of a selected pixel, and outputting a reflected data value of the selected pixel, wherein the adjacent pixel is adjacent to the selected pixel, and the gain value depends on a data value of the selected pixel, wherein the gain value is determined based on a brightness value of the selected pixel, wherein the brightness value and the gain value of the selected pixel are determined by a graph in which the brightness value and the gain value have an inverse proportional relationship, and wherein the graph of the inverse proportional relationship has a shape which is adjusted based on an average brightness value and a standard deviation of the data values.