The present invention is to provide an optical add/drop multiplexing device capable of realizing a configuration in which many transponders can be connected at low cost.
An optical add/drop multiplexing device 30A includes branch function units 34a to 34d connected to each of WSSs 22a to 22d connected to respective routes 1 to D having a plurality of optical fibers and dropping optical signals having a plurality of wavelengths among the optical signals having the respective wavelengths transmitted by wavelength division multiplexing. The optical add/drop multiplexing device includes C-function units 35a to 35d configured to transmit the optical signals branched by the branch function units 34a to 34d to a plurality of transponders. The optical add/drop multiplexing device includes optical couplers 36a to 36d each connected between a set of K WSSs 22c and 22d each having 1 input and M outputs and one of the branch function units 34a, K being a number of 2 or more, the number of optical couplers being equal to or greater than a positive integer obtained by dividing a numerical value D(M−D) by K, the numerical value being obtained by multiplying the number (M−D) of optical signals dropped by each of the WSSs 22c and 22d by the number D of all of the WSSs 22a to 22d. The optical coupler 36a couples optical signals of different wavelengths dropped by a set of WSSs into one optical signal and outputs the coupled optical signal to the branch function unit 34a.