This paper considers the estimation of complex dielectric constant of metal backed material by using spatial cylindrical standing wave. A line source is located near the metal backed material and electric field strength between the source and the material is obtained by microwave experiment. The electric field strength is analyzed theoretically by changing the electric parameters of the material: dielectric constant and conductivity. The electric parameters for which the electric field strength by theoretical analysis is good agreement with that by experiment are estimated to be the electric parameters of the material. The estimated parameters are also compared with those obtained by the coaxial probe method.