A wavelength division multiplexing filter according to the present invention is a wavelength division multiplexing filter having a variable filter shape. The wavelength division multiplexing filter has a resolution equal to 2N times (N being an integer, where N≧1) a reference frequency interval and has frequency arrangement of center frequencies of frequency slots corresponding to the 2N-times resolution be offset by half a cycle with respect to the frequency arrangement of standard wavelength grids having a frequency interval equal to ½N of the reference frequency interval.