<p>The theory and practice of vibration diagnostics of crack-like damages are mainly developed in the line of relatively simple mechanical systems: cantilever rod, simply-supported beam or shaft, etc. The evolution of vibration damage diagnostics methods is currently central for more complicated mechanical systems. For example, it has no alternatives when it comes to steam turbines due to the complexity of their design, inaccessibility of their most loaded elements to measure vibrations in operation, and limited number of natural sources of vibrations whereby the changes in their parameters can be used for crack diagnostics. The efficiency of vibration diagnostics for edge and annular cracks in a five-support shaft is evaluated. For solving the above problem, an analytical-computational model of this shaft is proposed to determine the natural frequencies of bending vibrations. In general, the changing intensity of natural frequencies is shown to be governed by the crack shape and its parameters (depth and location). The efficiency of vibration diagnostics is proposed to be evaluated after a procedure for determining crack parameters wherein examined natural frequencies vary by a predetermined value, which should exceed the error of the experimental frequency measurement technique. The vibration diagnostics efficiency of crack-like damages based on the change in the natural frequencies of the first four shaft vibration modes was analyzed. It shows promise for such diagnostics to analyze complicated multi-support mechanical systems under rather exacting requirements as regards the error of the experimental technique and on condition that the change in the frequency of several vibration modes is used as an information damage characteristic.</p>