  We study "buried" active galactic nuclei (AGNs) almost fully covered by
circumnuclear material in ultra-/luminous infrared galaxies (U/LIRGs), which
show weak ionized lines from narrow line regions. Employing an indicator of [O
IV] 25.89-um or [Ne V] 14.32-um line to 12-um AGN luminosity ratio, we find 17
buried AGN candidates that are [O IV]-weak ($L_{\rm [O\,IV]}$/$L_{\rm 12,AGN}
\leq -$3.0) or [Ne V]-weak ($L_{\rm [Ne\,V]}$/$L_{\rm 12,AGN} \leq -$3.4) among
30 AGNs in local U/LIRGs. For the [O IV]-weak AGNs, we estimate their covering
fractions of Compton-thick (CT; $N_{\rm H} \geq 10^{24}$ cm$^{-2}$) material
with an X-ray clumpy torus model to be $f^{\rm (spec)}_{\rm CT} = 0.55\pm0.19$
on average. This value is consistent with the fraction of CT AGNs ($f^{\rm
(stat)}_{\rm CT} = 53\pm12$%) among the [O IV]-weak AGNs in U/LIRGs and much
larger than that in Swift/BAT AGNs ($23\pm6$%). The fraction of [O IV]-weak
AGNs increases from $27^{+13}_{-10}$% (early) to $66^{+10}_{-12}$% (late
mergers). Similar results are obtained with the [Ne V] line. The [O IV] or [Ne
V]-weak AGNs in late mergers show larger $N_{\rm H}$ and Eddington ratios
($\lambda_{\rm Edd}$) than those of the Swift/BAT AGNs, and the largest $N_{\rm
H}$ is $\gtrsim$10$^{25}$ cm$^{-2}$ at ${\log}\lambda_{\rm Edd} \sim -$1, close
to the effective Eddington limit for CT material. These suggest that (1) the
circumnuclear material in buried AGNs is regulated by the radiation force from
high-$\lambda_{\rm Edd}$ AGNs on the CT obscurers, and (2) their dense material
with large $f^{\rm (spec)}_{\rm CT}$ ($\sim 0.5 \pm 0.1$) in U/LIRGs is a
likely cause of a unique structure of buried AGNs, whose amount of material may
be maintained through merger-induced supply from their host galaxies.
