  We report on X-ray spectral evolution of the nearby Type IIn supernova (SN)
2005ip, based on Chandra and Swift observations covering from ~1 to 6 years
after the explosion. X-ray spectra in all epochs are well fitted by a thermal
emission model with kT > 7 keV. The somewhat high temperature suggests that the
X-ray emission mainly arises from the circumstellar medium heated by the
forward shock. We find that the spectra taken 2-3 years since the explosion are
heavily absorbed N_H ~ 5e22 cm^{-2}, but the absorption gradually decreases to
the level of the Galactic absorption N_H ~ 4e20 cm^{-2} at the final epoch.
This indicates that the SN went off in a dense circumstellar medium and that
the forward shock has overtaken it. The intrinsic X-ray luminosity stays
constant until the final epoch when it drops by a factor of ~2. The intrinsic
0.2-10 keV luminosity during the plateau phase is measured to be ~1.5e41 erg/s,
ranking SN 2005ip as one of the brightest X-ray SNe. Based on the column
density, we derive a lower-limit of a mass-loss rate to be M_dot ~ 0.015
(V_w/100 km/s) M_sun/yr, which roughly agrees with that inferred from the X-ray
luminosity, M_dot ~ 0.02 (V_w/100 km/s) M_sun/yr, where V_w is the
circumstellar wind speed. Such a high mass-loss rate suggests that the
progenitor star had eruptive mass ejections like a luminous blue variable star.
The total mass ejected in the eruptive period is estimated to be ~15 M_sun,
indicating that the progenitor mass is greater than ~25 M_sun.
