  We present an analysis of X-ray observations of the Ultraluminous X-ray
source (ULX) in IZw18 based on archival data taken with Chandra, XMM-Newton,
and Suzaku. This ULX is considered to be an intermediate-mass black hole
candidate simply because it is in the lowest metallicity environment among
ULXs, where formation of heavy black holes is facilitated. However, actual
study of the ULX based on observations spanning for a long period has been too
limited to determine its nature. In this study, we investigate the spectral
evolution of the ULX up to 2014, combining the previously-unpublished Suzaku
data with those from the other two satellites. We derive a positive correlation
of $L\propto T_{\rm in}^{2.1\pm0.4}$ between the bolometric luminosity $L$ and
inner-disk temperature $T_{\rm in}$ on the basis of the multi-color
disk-blackbody model, where we exclude the Chandra data, which has the lowest
luminosity and systematic residuals in the fitting. The nominal relation
$L\propto T_{\rm in}^{4}$ for the standard disk is rejected at a significance
level of 1.5 %. These results suggest that the ULX was in the slim-disk state
during these observations except at the time of the Chandra observation, in
which the ULX was likely to be in a different state. The apparent inner-disk
radius appears negatively correlated with the inner-disk temperature. Moreover,
we find a radial dependence of the disk temperature of $T (r)\propto r^{-p}$
with $p<0.75$, which also supports the hypothesis that the ULX has a slim disk.
In consequence, the IZw18 ULX is most likely to be powered by a stellar-mass
compact object in supercritical accretion.
