  We have been developing monolithic active pixel sensors, known as Kyoto's
X-ray SOIPIXs, based on the CMOS SOI (silicon-on-insulator) technology for
next-generation X-ray astronomy satellites. The event trigger output function
implemented in each pixel offers microsecond time resolution and enables
reduction of the non-X-ray background that dominates the high X-ray energy band
above 5--10 keV. A fully depleted SOI with a thick depletion layer and back
illumination offers wide band coverage of 0.3--40 keV. Here, we report recent
progress in the X-ray SOIPIX development. In this study, we achieved an energy
resolution of 300~eV (FWHM) at 6~keV and a read-out noise of 33~e- (rms) in the
frame readout mode, which allows us to clearly resolve Mn-K$\alpha$ and
K$\beta$. Moreover, we produced a fully depleted layer with a thickness of
$500~{\rm \mu m}$. The event-driven readout mode has already been successfully
demonstrated.
