A theoretical study of plane waves in hypoplastic medium is presented using a ray method of geometrical seismology. Dynamical equations for relative small disturbance about an initial state are derived from general equations of hypoplasticity. We consider the hypoplastic equations with intergranular strain and assume that the initial state and the density of medium are inhomogeneous. In this case using of the ray method is the most efficient. The solutions for the components of rate vector or stress are found in the form of ray series. The coefficients of these series satisfy on order differential equations of the transfer which are solved analytical in case when void ratio is less then unit. The velocities of quasi-longitudinal and quasi-transverse waves in a hypoplastic medium with intergranular strain depend on a kind of the loading and the unloading and they are usually less than the velocities of longitudinal and transverse waves in elastic skeleton.