[en] We developed a trigger system for the measurement of proton-induced rare hadronic reactions around the beam kinetic energy T_p=400MeV based on highly segmented trigger scintillation detectors and programmable logic modules. The trigger system was designed to enhance events with the negative-pion production by the difference of the curvatures of the particle tracks in a magnetic field. Since the production cross-section of the negative-pion by the proton-induced reactions was smaller by about 3 orders of magnitude than the total cross-section around the beam energy, we expected large reduction of the trigger rate by the negative-pion selection. The construction of the trigger system was not trivial due to a large detector acceptance that was inevitable to measure the rare reactions. The performance of the trigger system was evaluated by using T_p=430MeV proton beams. An excellent reduction, more than 2 orders of magnitude reduction, of the background events was achieved