  We investigate measurement of electron transport in quantum dot systems by
using single-electron transistor as a noninvasive detector. It is demonstrated
that such a detector can operate in the ``negative-result measurement'' regime.
In this case the measured current is not distorted, providing that it is a
non-coherent one. For a coherent transport, however, the possibility of
observing a particular state out of coherent superposition leads to distortion
of a measured current even in the ``negative-result measurement'' regime. The
corresponding decoherence rate is obtained in the framework of quantum rate
equations.
