  We present an analysis of the albedo properties of main belt asteroids
detected by the All-Sky Survey of the infrared satellite AKARI. The
characteristics of 5120 asteroids detected by the survey, including their sizes
and albedos, were cataloged in the Asteroid Catalog Using AKARI (AcuA). Size
and albedo measurements were based on the Standard Thermal Model, using inputs
of infrared fluxes and absolute magnitudes. Main belt asteroids, which account
for 4722 of the 5120 AcuA asteroids, have semimajor axes of 2.06 to 3.27 AU.
AcuA provides a complete data set of all main belt asteroids brighter than the
absolute magnitude of H < 10.3, which corresponds to the diameter of d > 20 km.
We confirmed that the albedo distribution of the main belt asteroids is
strongly bimodal as was already known from the past observations, and that the
bimodal distribution occurs not only in the total population, but also within
inner, middle, and outer regions of the main belt. We found that the small
asteroids have much more variety in albedo than the large asteroids. In spite
of the albedo transition process like space weathering, the heliocentric
distribution of the mean albedo of asteroids in each taxonomic type is nearly
flat. The mean albedo of the total, on the other hand, gradually decreases with
an increase in semimajor axis. This can be explained by the compositional ratio
of taxonomic types; that is, the proportion of dark asteroids such as C- and
D-types increases, while that of bright asteroids such as S-type decreases,
with increasing heliocentric distance. The heliocentric distributions of
X-subclasses: E-, M-, and P-type, which can be divided based on albedo values,
are also examined. P-type, which is the major component in X-types, are
distributed throughout the main belt regions, and the abundance of P-type
increases beyond 3 AU. This distribution is similar to that of C- or D-types.
