  Measurements of the position of a relativistic particle is considered in the
framework of the Restricted-Path-Integral (RPI) approach. The amplitude
describing such a measurement is shown to be exponentially small outside the
light cone of the space-time point corresponding to the measurement output, in
a qualitative agreement with the Hellwig and Kraus' postulate of relativistic
state reduction. Theory of the measurement including the probability
distribution for different measurement outputs is suggested. It is shown that
correct theory does not exist (for arbitrary initial states) if the error of
the measurement is less than the Compton length. The physical reason is that
the picture of measurement is destroyed in this case by pair creation.
