It is known that the difference in shear modulus for normal tissues and for pathological ones can reach two or three orders. This is the basis for the development of a very sensitive medical diagnostics method. Shear stresses inside of tissue can be excited by a focused ultrasound. The efficiency of this process is the main problem. The phenomenon of shear wave excitation by a focused acoustical beam has been studied experimentally and theoretically. In the experiment a shear wave has been excited in rubberlike material (transparent polymer) by powerful focused ultrasonic beam in a pulsed regime. The registration of the shear deformation has been performed by means of an optical beam, which was focused at a target in the thickness of polymer sample. Temporal profiles of a shear wave have been registered at different distances from the focal region. The measured value of shear wave velocity was in a good agreement with its theoretical prediction. A theoretical model of shear wave generation by a beam of a longitudinal wave has been developed. An analytical expression for the pulse response function was derived which allows one to study the spatial and temporal characteristics of the shear wave.