An accelerator experiment was performed using a low-energy antiproton beam to measure antiproton detection efficiency of BESS, a balloon-borne spectrometer with a superconducting solenoid. Measured efficiencies showed good agreement with calculated ones derived from the BESS Monte Carlo simulation based on GEANT/GHEISHA. With detailed verification of the BESS simulation, the relative systematic error of detection efficiency derived from the BESS simulation has been determined to be +/-5%, compared with the previous estimation of +/- 15% which was the dominant uncertainty for measurements of cosmic-ray antiproton flux. (C) 2002 Elsevier Science B.V. All rights reserved.