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The intermediate polar GK Per exhibited a dwarf nova outburst in March-April2015. Suzaku X-ray telescope serendipitously captured the onset of the outburstduring its pre-scheduled pointing observation spanning four days. In thispaper, 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-raycount rates in outburst with a WD spin period of P_WD=351.4+/-0.5 s. Themodulation is also detected in the hard X-ray band (16-60 keV), and spectralmodeling of the absorption suggests obscuration by a dense absorption with aline-of-sight column density of N_H>10^23 cm^-2. A complex time evolution ofspin 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 thepulse shape significantly changes epoch by epoch.  Spectral fitting in the Fe Kalpha band revealed an increase of thefluorescent line equivalent width, from ~80 eV (quiescent) to ~140 eV(outburst). The equivalent widths of He-like and H-like Fe Kalpha areconsistent with being constant at ~40 eV in the two states. Broad-band spectralfitting in the 2-60 keV band resulted in a sub-solar Fe abundance of ~0.1 Z_sunand the maximum plasma temperature kT_max~50-60 keV when the isobaric coolingflow model was applied. Based on the very small temperature change against a6-7-times increased accretion rate, the accretion geometry in early outburst isdiscussed.

GK Per 2015 dwarf nova covered by Suzaku