  We study the gluon propagator in the Landau gauge in SU(3) lattice QCD at
$\beta$=5.7, 5.8 and 6.0 at the quenched level. The Euclidean Landau-gauge
gluon propagator $D(r)\equiv D_{\mu\mu}^{aa}(x)/24$ is found to be well
described by four-dimensional Yukawa-type function $e^{-mr}/r$ in the infrared
and intermediate region of $r \equiv (x_\mu x_\mu)^{1/2}$ = 0.1 $\sim$ 1.0fm.
The infrared effective gluon mass is obtained as $m \simeq$ 600MeV. Associated
with the 4D 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.
Remarkably, the obtained gluon spectral function $\rho(\omega)$ is almost
negative definite, except for a positive $\delta$-functional peak at
$\omega=m$. Since the Yukawa-type propagation indicates a three-dimensional
space-time character, we consider a hypothesis of an effective dimensional
reduction by generalized Parisi-Sourlas mechanism in a stochastic
color-magnetic vacuum of infrared QCD.
