  We postulate a master equation, written in the language of creation and
annihilation operators, as a candidate for unambiguous quantum mechanical
description of unstable particles. We have found Kraus representation for the
evolution driven by this master equation and study its properties. Both
Schroedinger and Heisenberg picture of the system evolution are presented. We
show that the resulting time evolution leads to exponential decay law.
Moreover, we analyse mixing of particle flavours and we show that it can lead
to flavour oscillation phenomenon.
