To provide an arithmetic unit having an increased operation speed by a relatively simple structure by using the physical properties of light.

This unit is equipped with (n) light-light switched and (n) spectroscope groups corresponding to the light-light switches; and (n) spectroscopes which transmit only light of a previously selected wavelength are arranged in the individual spectroscope groups and the light beams transmitted by the individual spectroscopes are defined as numbers 0, 1, 2,..., n-1. A light source and the (n) light-light switched are connected by optical fibers, which extend from the (n) light-light switched into the corresponding spectroscope groups. Then the (y)th spectroscope of an (x)th spectroscope group is connected to an (x+y)th light-light switch (when (x+y)≤n) or an (x+y-n)th light-light switch (when (x+y)>n) by the optical fibers and the optical fibers connecting the spectroscopes and light-light switched branch off and are connected to output terminals defined by 'x-1' (where x=1, 2, 3,..., n and y=0, 1, 2,..., n-1).