  A theoretical study is reported on the origin of extremely high upper
critical field $\sim$70T observed in UTe$_2$ with the transition temperature
T$_{\rm c}$=1.6K-2K, far exceeding the conventional orbital depairing limit set
by the Fermi velocity and T$_{\rm c}$ for a superconductor (SC) in the clean
limit. We investigate possible violation of the orbital limit in terms of a
spin-triplet nonunitary state, which is effectively coupled to the underlying
magnetization induced by external field. This in turn produces the reduced
internal field by cancelling it via magnetization. We formulate a theory within
Ginzburg-Landau framework to describe this orbital limit violation and analyze
experimental data on the upper critical fields for various field orientations
in UTe$_2$. It is pointed out that the orbital limit violation for a
spin-triplet SC together with the Pauli-Clogston limit violation for a
spin-singlet SC constitutes a complete and useful framework to examine the high
field physics in superconductors in the clean limit.
