Near-infrared (IR) diffuse sky brightness is contributed from zodiacal light (ZL),
integrated starlight (ISL), diffuse Galactic light (DGL), and diffuse isotropic light
including extragalactic background light (EBL). The diffuse near-IR radiation was
studied by all-sky maps obtained with the Diffuse Infrared Background Experiment (DIRBE)
onboard Cosmic Background Explorer (COBE). In the previous analysis, however, large ISL
uncertainty in a star-counts model caused non-detection of the DGL and diffuse isotropic
light. In this thesis, the diffuse near-IR radiation is reanalyzed by improving the ISL
evaluation.

In the present analysis, DIRBE all-sky maps at 1.25, 2.2, 3.5, and 4.9 um are used as
total diffuse near-IR brightness. To improve the ISL evaluation, star catalogs based on
Two Micron All-Sky Survey (2MASS) and Wide-field Infrared Survey Explorer (WISE) are
used. As a result, the DIRBE sky brightness is decomposed into the ZL, DGL, ISL, and
diffuse isotropic light in high Galactic latitudes.

The near-IR DGL consists of scattered light and thermal emission from interstellar
dust. Using the DGL result at 3.5 um, mass fraction of very small grains and polycyclic
aromatic hydrocarbon is constrained to be 2%-8%. The scattered light observed at 1.25
and 2.2 um is redder than that expected from a current dust model. This trend may
suggest the presence of larger dust grains in the diffuse interstellar medium.

At 1.25 and 2.2 um, intensity ratios of the DGL to 100 um emission are found to increase
toward low Galactic latitudes. The observed latitude dependence is steeper than a
scattered light model in which the scattering phase function reproduces a current dust
model.  This may imply stronger forward scattering by larger grains, which is consistent
with the implication from the redder spectrum of the scattered light.

The diffuse isotropic light obtained at 1.25 and 2.2 um is several times larger than
integrated galaxy light and EBL limit derived from high-energy gamma-ray observation,
indicating local origin of the excess light. To explain the excess, hypothetical
interplanetary dust distributed around the Sun is introduced. Additional 5%-10% density
relative to the known interplanetary dust can explain the excess light. In the future,
observation outside the Earth orbit will be useful to confirm the origin of the diffuse
isotropic light.