  It is commonly believed that the most general type of a quantum-mechanical
measurement is one described by a positive-operator valued measure (POVM). In
the present paper, this statement is proven for any measurements on quantum
systems with a finite-dimensional state space. The proof of POVM nature of an
arbitrary measurement is carried out using a purely operational approach, which
is fully ignorant about what is inside a measurement apparatus. The suggested
approach gives also an opportunity to derive the Born rule.
