  A deep X-ray observation of the unidentified very high energy (VHE) gamma-ray
source HESS J1702-420, for the first time, was carried out by Suzaku. No bright
sources were detected in the XIS field of view (FOV) except for two faint
point-like sources. The two sources, however, are considered not to be related
to HESS J1702-420, because their fluxes in the 2-10 keV band (~ 10^-14 erg s^-1
cm^-2) are ~ 3 orders of magnitude smaller than the VHE gamma-ray flux in the
1-10 TeV band (F_{TeV} = 3.1 x 10^-11 erg s^-1 cm^-2). We compared the energy
spectrum of diffuse emission, extracted from the entire XIS FOV with those from
nearby observations. If we consider the systematic error of background
subtraction, no significant diffuse emission was detected with an upper limit
of F_X <2.7 x 10^-12 erg s^-1 cm^-2 in the 2-10 keV band for an assumed
power-law spectrum of \Gamma=2.1 and a source size same as that in the VHE
band. The upper limit of the X-ray flux is twelve times as small as the VHE
gamma-ray flux. The large flux ratio (F_{TeV}/F_X) indicates that HESS
J1702-420 is another example of a "dark" particle accelerator. If we use a
simple one-zone leptonic model, in which VHE gamma-rays are produced through
inverse Compton scattering of the cosmic microwave background and interstellar
far-infrared emission, and the X-rays via the synchrotron mechanism, an upper
limit of the magnetic field (1.7 \mu G) is obtained from the flux ratio.
Because the magnetic field is weaker than the typical value in the Galactic
plane (3-10 \mu G), the simple one-zone model may not work for HESS J1702-420
and a significant fraction of the VHE gamma-rays may originate from protons.
