Physical background of the principle of equivalence

In this paper we assert that Einstein's assumption of the validity of special relativity in an infinitely small domain of space-time cannot be accepted if the dilatational degree of freedom exists as a natural key in explaining the existence of gravitation. In this case the equality of inertial and gravitational force holds through the whole structure of space-time but their intensities change continuously by following the behaviour of function which is responsible for reproduction of time and length units from point to point. In addition we discuss the way in which the pseudo-tensor of gravitational field was obtained.