  A general discussion is given for first-kind (FK) and quantum non-demolition
(QND) measurements. The general conditions for these measurements are derived,
including the most general one (called the weak condition), an intermediate
one, and the strongest one. The weak condition indicates that we can realize a
FK or QND measuring apparatus of wide classes of observables by allowing the
apparatus to have a finite response range. A recently-proposed QND
photodetector using an electron interferometer is an example of such apparatus.
