  The indeterminacy inherent in quantum measurement is an outstanding character
of quantum theory, which manifests itself typically in Heisenberg's
error-disturbance uncertainty relation. In the last decade, Heisenberg's
relation has been generalized to hold for completely general quantum
measurements. Nevertheless, the strength of those relations has not been
clarified yet for mixed quantum states. Recently, a new error-disturbance
uncertainty relation (EDUR), stringent for generalized input states, has been
introduced by one of the present authors. A neutron-optical experiment is
carried out to investigate this new relation: it is tested whether error and
disturbance of quantum measurements disappear or persist in mixing up the
measured ensemble. Our results exhibit that measurement error and disturbance
remain constant independent of the degree of mixture. The tightness of the new
EDUR is confirmed, thereby validating the theoretical prediction.
