  Irreversibility in quantum measurements is considered from the point of
quantum information theory. For that purpose the information transfer between
the measured object S and measuring system O is analyzed. It's found that due
to the principal constraints of quantum-mechanical origin, the information
about the purity of S state isn't transferred to O during the measurement of
arbitraryS observable V. Consequently O can't discriminate the pure and mixed S
ensembles with the same <V>. As the result, the random outcomes should be
detected by O in V measurement for S pure ensemble of V eigenstate
superposition. It's shown that the outcome probabilties obey to Born rule. The
influence of O decoherence by its environment is studied, however the account
of its effects doesn't change these results principally.
