  The uncertainty relation, which displays an elementary property of quantum
theory, was originally described by Heisenberg as the relation between error
and disturbance. Ozawa presented a more rigorous expression of the uncertainty
relation, which was later verified experimentally. Nevertheless, the operators
corresponding to error and disturbance should be measurable in the identical
state if we follow the presupposition of Heisenberg's thought experiment. In
this letter, we discuss simultaneous measurability of error and disturbance and
present a new inequality using error and disturbance in the identical state. A
testable example of this inequality is also suggested.
