  We analyze the precursor of a Forbush Decrease (FD) observed with the Global
Muon Detector Network on December 14, 2006. An intense geomagnetic storm is
also recorded during this FD with the peak Kp index of 8+. By using the
"two-dimensional map" of the cosmic ray intensity produced after removing the
contribution from the diurnal anisotropy, we succeed in extracting clear
signatures of the precursor. A striking feature of this event is that a weak
loss-cone signature is first recorded more than a day prior to the Storm Sudden
Commencement (SSC) onset. This suggests that the loss-cone precursor appeared
only 7 hours after the Coronal Mass Ejection (CME) eruption from the Sun, when
the Interplanetary (IP) shock driven by the Interplanetary Coronal Mass
Ejection (ICME) located at 0.4 AU from the Sun. We find the precursor being
successively observed with multiple detectors in the network according to the
Earth's spin and confirmed that the precursor continuously exists in space. The
long lead time (15.6 hours) of this precursor which is almost twice the typical
value indicates that the IMF was more quiet in this event than a typical power
spectrum assumed for the IMF turbulence. The amplitude (-6.45 %) of the
loss-cone anisotropy at the SSC onset is more than twice the FD size,
indicating that the maximum intensity depression behind the IP shock is much
larger than the FD size recorded at the Earth in this event. We also find the
excess intensity from the sunward IMF direction clearly observed during ~10
hours preceding the SSC onset. It is shown that this excess intensity is
consistent with the measurement of the particles accelerated by the head-on
collisions with the approaching shock. This is the first detailed observation
of the precursor due to the shock reflected particles with muon detectors.
