the wavelength monitoring circuit includes:

a first split circuit that splits an input optical signal into two;

a first optical delay circuit that applies a delay time difference to the two split optical signals;

second and third split circuits that each further split, into two, a different one of the two optical signals to which the delay time difference has been applied;

a second optical delay circuit that applies a delay time difference to each of sets of optical signals derived from four optical signals resulting from the two-way splits by the second and the third split circuits, the sets each being made up of two optical signals to which mutually-different delay time differences have been applied by the first optical delay circuit; and

first and second two-input two-output optical multiplexer/demultiplexer circuits that each output a result of applying multiplexing interference to the optical signals to which the delay time difference has been applied,

a difference between the delay time differences applied by the second optical delay circuit to a first set of optical signals and to a second set of optical signals can be expressed as λ/4c where λ is any of wavelengths in an optical signal wavelength band being monitored, while c is a speed of light,

the wavelength monitoring apparatus further includes a photoelectric conversion element that performs a photoelectric conversion on the optical signals output from the wavelength monitoring circuit so as to output electrical signals, and

the wavelength monitoring apparatus is configured to obtain a wavelength of the optical signal input to the wavelength monitoring circuit, by calculating a ratio between electrical outputs in the first set of optical signals and a ratio between electrical outputs in the second set of optical signals that are output from the photoelectric conversion element and referring to a correspondence table indicating wavelengths of optical signals input to the wavelength monitoring circuit and ratios between electrical outputs,

wherein the photoelectric conversion element that performs the photoelectric conversion on the optical signals output from the wavelength monitoring circuit so as to output the electrical signals is not provided with a photoelectric conversion element that receives an input of one optical signal in the second set of optical signals output from the second two-input two-output optical multiplexer/demultiplexer circuit, and

the wavelength monitoring apparatus has one of following configurations:

(a) being configured to obtain the wavelength of the optical signal input to the wavelength monitoring circuit by using, in place of the electrical output of one of the optical signals in the second set of optical signals, a value obtained by subtracting the electrical output being output from the photoelectric conversion element receiving an input of the other optical signal in the second set of optical signals, from a sum of the electrical outputs being output from the photoelectric conversion element receiving an input of the first set of optical signals; and

(b) being configured to obtain the wavelength of the optical signal input to the wavelength monitoring circuit, by calculating a ratio between one of the electrical outputs being output from the photoelectric conversion element receiving an input of the first set of optical signals and a sum of the electrical outputs being output from the photoelectric conversion element receiving an input of the first set of optical signals; and another ratio between the electrical output being output from the photoelectric conversion element receiving an input of the other optical signal in the second set of optical signals and a sum of the electrical outputs being output from the photoelectric conversion element receiving an input of the first set of optical signals and referring to the correspondence table indicating the wavelengths of the optical signals input to the wavelength monitoring circuit and the ratios between the electrical outputs.