  The three-dimensional (3D) electronic structure of the hidden order compound
URu$_2$Si$_2$ in a paramagnetic phase was revealed using a 3D angle-resolved
photoelectron spectroscopy where the electronic structure of the entire
Brillouin zone is obtained by scanning both incident photon energy and
detection angles of photoelectrons. The quasi-particle bands with enhanced
contribution from the $\mathrm{U}~5f$ state were observed near $E_\mathrm{F}$,
formed by the hybridization with the $\mathrm{Ru}~4d$ states. The energy
dispersion of the quasi-particle band is significantly depend on $k_z$,
indicating that they inherently have a 3D nature. The band-structure
calculation qualitatively explain the characteristic features of the band
structure and Fermi surface although the electron correlation effect strongly
renormalizes the quasi-particle bands. The 3D and strongly-correlated nature of
the quasi-particle bands in URu$_2$Si$_2$ is an essential ingredient for
modeling its hidden-order transition.
