A new acoustical method of determination of porosity of the solid saturated by a liquid is proposed. The method is based on the multiple application of the focused Gaussian beam of the quasi-harmonic sound waves locally irradiating a plane boundary interface between a fluid phase and porous medium in the mass transfer process, in particular, drying of the materials. The scanning of the sound beam on the boundary surface allows a possibility of the non-specular reflection. It is shown that maximum value of the wave power in the non-specular reflected sound can be achieved, when an angle of the incidence wave coincidences with a critical angle of the Rayleigh surface wave excitation in the porous elastic solid. A solution of the dispersion equation of this problem is connected with the Rayleigh critical angle, from one hand, and porosity parameters of the reflecting body, from other hand. It allows immediately to determinate the initial porosity of the medium from the amplitude of non-specular reflected sound for each of the discrete acts of the sound irradiation. Numerical calculations demonstrate the effect of the non-specular reflection effect for a system: water-sandstone saturated by kerosene.