  We performed the Shubnikov-de Haas (SdH) experiments of the low carrier heavy
fermion compound URu2Si2 at high fields up to 34T and at low temperatures down
to 30mK. All main SdH branches named alpha, beta and gamma were observed for
all the measured field-directions (H // [001] -> [100], [100] -> [110] and
[001] -> [110]), indicating that these are attributed to the closed Fermi
surfaces with nearly spherical shapes. Anomalous split of branch alpha was
detected for the field along the basal plane, and the split immediately
disappears by tilting the field to [001] direction, implying a fingerprint of
the hidden order state. High field experiments reveal the complicated
field-dependence of the SdH frequencies and the cyclotron masses due to the
Zeeman spin-splitting associated with the Fermi surface reconstruction in the
hidden order state with small carrier numbers. A new SdH branch named omega
with large cyclotron mass of 25m0 was detected at high fields above 23T close
to the hidden order instabilities.
