A frequency assignment method for selecting a frequency width used on a route connecting between a start point and an end point when the start point and the end point of an optical signal are supplied in a photonic network including an optical node that includes an optical switch for switching the optical signal without electrically terminating the optical signal is disclosed. The frequency assignment method includes steps of: obtaining a correlation amount of use state of wavelength or frequency between adjacent links by referring to a route calculation result; determining a fixed frequency width or variable frequency width to be set for a communication route based on the correlation amount; and assigning the fixed frequency width or the variable frequency width on the route.