  Nonperturbative properties of gluons are studied in SU(3) lattice QCD at the
quenched level. The first subject is a functional-form analysis of the gluon
propagator $D_{\mu\nu}^{ab}(x)$ in the Landau gauge. We find that the gluon
propagator $D_{\mu\mu}^{aa}(r)$ obtained in lattice QCD is well described by
the four-dimensional (4D) Yukawa-type function $e^{-mr}/r$ with $m \simeq
600$MeV for the Euclidean 4D distance $r = 0.1 \sim 1.0$ fm. In momentum space,
the gluon propagator $\tilde D_{\mu\mu}^{aa}(p^2)$ ($p= 0.5 \sim 3$ GeV) is
found to be well approximated with a new-type propagator of $(p^2+m^2)^{-3/2}$,
which corresponds to 4D Fourier image of the Yukawa-type function. Associated
with the Yukawa-type gluon propagator, we derive analytical expressions for the
zero-spatial-momentum propagator $D_0(t)$, the effective mass $M_{\rm eff}(t)$,
and the spectral function $\rho(\omega)$ of the gluon field. The mass parameter
$m$ turns out to be the infrared effective mass of gluons. The obtained gluon
spectral function $\rho(\omega)$ is almost negative-definite for $\omega >m$,
except for a positive $\delta$-functional peak at $\omega=m$. The second
subject is a lattice-QCD determination of the relevant gluonic
momentum-component for color confinement. As a remarkable fact, the string
tension is found to be almost unchanged even after cutting off the
high-momentum gluon component above 1.5 GeV in the Landau gauge. In fact, the
relevant gluonic scale for color confinement is concluded to be below 1.5 GeV.
