There is provided a network management device to manage a network on which an existing optical path is operated, the network management device including: a first receiver configured to receive data related to a measurement result of signal quality of a first optical signal having a first wavelength on the existing optical path; a second receiver configured to receive a request for adding a second optical signal having a second wavelength to the existing optical path; at least one memory configured to store a procedure; and at least one processor configured to execute the procedure of estimating a variation quantity in the signal quality of the first optical signal when the second optical signal is added to the existing optical path based on the data related to the measurement result, when the request is received, and determining whether to add the second optical signal based on results of the estimating.