The paper discusses principles for measuring density of wood on the basis of using the polarity effect for electromagnetic waves in anisotropic and isotropic substances. Relationships between velocity of propagation of orthogonally polarized waves and permittivity of substances measured are examined. Information parameters of polarized waves and their dependences on the parameters measured are identified. Errors in measurement results are analyzed and a method for controlling them is proposed. An application field for the solid material density measuring device is given.