A well known method of successive approsimations for obtaining principal frequencies of elastic systems with concertrated masses, i.e. of systems the approximative function of which for kinetic energy contains only squares of the generalized velocties has been generalized by the author in such a way as to be valid for any vibating system, that is to hold also for those systems the approximative function of which for the system kinetic energy is not aof a cannonic type, or in other words, the function that in addition to the squares of generalized velocities contains also products of various generalized velocites of the first order.