  The intermediate polar GK Per exhibited a dwarf nova outburst in March-April
2015. Suzaku X-ray telescope serendipitously captured the onset of the outburst
during its pre-scheduled pointing observation spanning four days. In this
paper, we present temporal and spectral analysis results of this outburst,
together with those from archival data of quiescent obtained in 2009 and 2014.
  Our temporal analysis confirmed previously reported spin modulation of X-ray
count rates in outburst with a WD spin period of P_WD=351.4+/-0.5 s. The
modulation is also detected in the hard X-ray band (16-60 keV), and spectral
modeling of the absorption suggests obscuration by a dense absorption with a
line-of-sight column density of N_H>10^23 cm^-2. A complex time evolution of
spin modulation profiles is seen; the spin minimum phase shifts from phase
~0.25 in the first half of the observation to ~0.65 in the second one, and the
pulse shape significantly changes epoch by epoch.
  Spectral fitting in the Fe Kalpha band revealed an increase of the
fluorescent line equivalent width, from ~80 eV (quiescent) to ~140 eV
(outburst). The equivalent widths of He-like and H-like Fe Kalpha are
consistent with being constant at ~40 eV in the two states. Broad-band spectral
fitting in the 2-60 keV band resulted in a sub-solar Fe abundance of ~0.1 Z_sun
and the maximum plasma temperature kT_max~50-60 keV when the isobaric cooling
flow model was applied. Based on the very small temperature change against a
6-7-times increased accretion rate, the accretion geometry in early outburst is
discussed.
