We performed the Shubnikov-de Haas (SdH) experiments of the low-carrier heavy-fermion compound URu ₂Si ₂ at high fields up to 34 T and at low temperatures down to 30 mK. All main SdH branches named α, β, and γ were observed for all the measured field-directions (H k ||001] → ||100], ||100] → ||110] and ||001] → ||110]), indicating that they are attributed to the closed Fermi surfaces with nearly spherical shapes. An anomalous split of branch α was detected for the field along the basal plane; the split immediately disappeared 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 ω with a large cyclotron mass of 25m ₀ was detected at high fields above 23 T close to hidden order instabilities.