  SS433, located at the center of the supernova remnant W50, is a close
proximity binary system consisting of a compact star and a normal star. Jets of
material are directed outwards from the vicinity of the compact star
symmetrically to the east and west. Non-thermal hard X-ray emission is detected
from lobes lying on both sides. Shock accelerated electrons are expected to
generate sub-TeV gamma rays through the inverse-Compton process in the lobes.
Observations of the western X-ray lobe region of SS433/W50 system have been
performed to detect sub-TeV gamma-rays using the 10m CANGAROO-II telescope in
August and September, 2001, and July and September, 2002. The total observation
times are 85.2 hours for ON source, and 80.8 hours for OFF source data. No
significant excess of sub-TeV gamma rays has been found at 3 regions of the
western X-ray lobe of SS433/W50 system. We have derived 99% confidence level
upper limits to the fluxes of gamma rays and have set constraints on the
strengths of the magnetic fields assuming the synchrotron/inverse-Compton model
for the wide energy range of photon spectrum from radio to TeV. The derived
lower limits are 4.3 microgauss for the center of the brightest X-ray emission
region and 6.3 microgauss for the far end from SS433 in the western X-ray lobe.
In addition, we suggest that the spot-like X-ray emission may provide a major
contribution to the hardest X-ray spectrum in the lobe.
