We report observation of isotropic interplanetary dust (IPD) by analyzing the
infrared (IR) maps of Diffuse Infrared Background Experiment (DIRBE) onboard
the Cosmic Background Explorer (COBE) spacecraft. To search for the isotropic
IPD, we perform new analysis in terms of solar elongation angle ($\epsilon$),
because we expect zodiacal light (ZL) intensity from the isotropic IPD to
decrease as a function of $\epsilon$. We use the DIRBE weekly-averaged maps
covering $64^\circ \lesssim \epsilon \lesssim 124^\circ$ and inspect the
$\epsilon$-dependence of residual intensity after subtracting conventional ZL
components. We find the $\epsilon$-dependence of the residuals, indicating the
presence of the isotropic IPD. However, the mid-IR $\epsilon$-dependence is
different from that of the isotropic IPD model at $\epsilon \gtrsim 90^\circ$,
where the residual intensity increases as a function of $\epsilon$. To explain
the observed $\epsilon$-dependence, we assume a spheroidal IPD cloud showing
higher density further away from the sun. We estimate intensity of the near-IR
extragalactic background light (EBL) by subtracting the spheroidal component,
assuming the spectral energy distribution from the residual brightness at
$12\,{\rm \mu m}$. The EBL intensity is derived as $45_{-8}^{+11}$,
$21_{-4}^{+3}$, and $15\pm3\,{\rm nWm^{-2}sr^{-1 } }$ at $1.25$, $2.2$, and
$3.5\,{\rm \mu m}$, respectively. The EBL is still a few times larger than
integrated light of normal galaxies, suggesting existence of unaccounted
extragalactic sources.