In the sending and receiving of a signal through an optical waveguide, an unknown substrate connected to the optical waveguide can be recognized. A signal processing apparatus 1A is provided with an optical waveguide 2A in which the light input from an arbitrary position on an outer circumference is output in an entire circumference direction at a level in accordance with a positional relation with the input location, a master substrate 3M arranged on the outer circumference of the optical waveguide 2A, a single or a plurality of slave substrates 3S arranged at any of a plurality of mount locations at different positions along the outer circumference of the optical waveguide 2A, in which as the light is output from A laser diodes 30S of all the slave substrates 3S connected to the optical waveguide 2A, from a level of a light transmitted through the optical waveguide 2A and input to the photo detector 31M of the master substrate 3M in accordance with the arrangement of the slave substrate 3S, the master substrate 3M recognizes the mount positions and number of the slave substrates 3S.