We present the results of a Suzaku spectroscopic study of soft extended X-ray emission in the H II region of M17. The spectrum of the extended emission was obtained with a high signal-to-noise ratio in a spatially resolved manner using the X-ray Imaging Spectrometer (XIS). We established that contamination by unresolved point sources, the Galactic Ridge X-ray emission, the cosmic X-ray background, and the local hot bubble emission is negligible in the background-subtracted XIS spectrum of the diffuse emission. Half a dozen emission lines were clearly resolved for the first time, including Kα lines of highly ionized O, Ne, and Mg as well as L-series complex of Fe at 0.51.5 keV. Based on diagnosis of these lines, we obtained the following results: (1) the extended emission is an optically thin thermal plasma, represented well by a single temperature of ∼ 3.0 ± 0.4MK, (2) the abundances of elements with emission lines in the diffuse spectrum are 0.1-0.3 solar, while those of bright discrete sources are 0.3-1.5 solar, (3) the metal abundances relative to each other in the diffuse emission are consistent with solar, except for a Ne enhancement of a factor of ∼ 2, (4) both the plasma temperature and the chemical composition of the diffuse emission show no spatial variation across the studied spatial scale of ∼ 5 pc. © 2008. Astronomical Society of Japan.