  We have been developing event-driven SOI Pixel Detectors, named `XRPIX'
(X-Ray soiPIXel) based on the silicon-on-insulator (SOI) pixel technology, for
the future X-ray astronomical satellite with wide band coverage from 0.5 keV to
40 keV. XRPIX has event trigger output function at each pixel to acquire a good
time resolution of a few $\mu \rm s$ and has Correlated Double Sampling
function to reduce electric noises. The good time resolution enables the XRPIX
to reduce Non X-ray Background in the high energy band above 10\,keV
drastically by using anti-coincidence technique with active shield counters
surrounding XRPIX. In order to increase the soft X-ray sensitivity, it is
necessary to make the dead layer on the X-ray incident surface as thin as
possible. Since XRPIX1b, which is a device at the initial stage of development,
is a front-illuminated (FI) type of XRPIX, low energy X-ray photons are
absorbed in the 8 $\rm \mu$m thick circuit layer, lowering the sensitivity in
the soft X-ray band. Therefore, we developed a back-illuminated (BI) device
XRPIX2b, and confirmed high detection efficiency down to 2.6 keV, below which
the efficiency is affected by the readout noise. In order to further improve
the detection efficiency in the soft X-ray band, we developed a
back-illuminated device XRPIX3b with lower readout noise. In this work, we
irradiated 2--5 keV X-ray beam collimated to 4 $\rm \mu m \phi$ to the sensor
layer side of the XRPIX3b at 6 $\rm \mu m$ pitch. In this paper, we reported
the uniformity of the relative detection efficiency, gain and energy resolution
in the subpixel level for the first time. We also confirmed that the variation
in the relative detection efficiency at the subpixel level reported by
Matsumura et al. has improved.
