  Based on the equivalence of quantum-mechanical reduced state and mixed state
which we demonstrate in this paper, we give a possible explanation of the
quantum measurement problem. We describe quantum measurement process as
two-step physical process: one microscopically controllable process which
produces entanglement, and one macroscopic process (uncontrollable
microscopically) which detects given states (or properties) of some microscopic
objects. With the explanation, we conclude that the measurement postulate is
just a corollary of the completeness of quantum state. Our explanation is
totally rooted in the unitarity of quantum mechanics, and does not need any
extension or modification of the formulation of orthodox quantum mechanics. We
also point out that quantum randomness comes from entanglement and wavefunction
collapse in quantum mechanics is subjective process.
