An enormous enhancement of antiproton production in deuteron-and alpha-induced reactions has been observed in the subthreshold energy region between 2 and 5 GeV/nucleon. Antiprotons produced at 5.1 degrees with a momentum range of between 1.0 and 2.5 GeV/c were measured by a beam-line spectrometer and identified by the time-of-flight method. The production cross sections in the deuteron-and a-induced reactions at an incident energy of 3.5 GeV/nucleon were 2 and 3 orders of magnitude larger than those in proton-induced reaction at the same energy. The enhancement in light-ion reactions could not be explained by the internal motion in the projectile and target nuclei. The target-mass dependence (C, Al, Cu and Pb) of the cross sections has also been studied. Further, the cross sections of pi and K productions were measured. (C) 1998 Elsevier Science B.V.