A beam test was executed to evaluate the behavior of the first prototype radiation-hard double-sided silicon microstrip sensor for the SDC silicon tracking system. Pions of 4 GeV/c in a test beamline at KEK illuminated three planes of detectors. The signals were amplified, shaped, and discriminated with TEKZ bipolar analog LSI's, and the on-off levels were sampled at 10MHz clock with CMOS digital LSI's, asynchronously with beam triggers. The detectors were rotated in null and 1.0 Tesla magnetic fields. The efficiencies were found to be 98approx.99%. The position resolutions were 12.5μm, where the multi-strip hit fraction was 30-40%. There was no essential difference in the performance of the p-and the n-sides. The multi-strip hit fraction showed a clear rotation and magnetic-field dependence. From the angles where the fractions were minimum in the 1T magnetic field, the Hall mobilities of the electrons and holes were obtained to be 1391±43 (electrons) and 325±30 (holes) cm2/Vs.