An efficient method of calculating the waveguide with two pairs of double ridges is suggested. It is based on the Green’s function method to solve the Helmhottz equations in three regions. Matrix equations are formed by using the Ritz’s functions and method of inner products. Solution of these equations yields the cut-off frequency and coefficients of the Ritz’s functions. Thus the fields can be completely described. The characteristic impedance of the waveguide system is calculated from the field expressions. Computed cut-off wavelengths are compared with experimental values given by Jull and computed fields compared with our measured results. Both show good consistency.