PURPOSE: To eliminate the need of a circuit which can withstand a large current by evading the influence of electrical noise on an event detecting signal by transmitting the detecting signal after converting the signal into an optical pulse and, at the same time, suppressing the wave height of a current sum signal in the final stage.

CONSTITUTION: When an event occurs in a measurement space in which a large number of detectors are arranged, detecting signals are sent from the detectors which detect the event to their corresponding measurement circuits 11. Each signal circuit 11-l to 11-m accumulate these output signals in each sample hold circuit 12 as electric charges and, when the charges exceeds the threshold of each comparator circuit 4, event occurring signals are outputted. The signals are transmitted to a trigger generation circuit 15 through each optical fiber cable 14 after the signals are converted into optical pulses at electrooptical converting sections 13 and then made incident to a photomultiplier tube 17. As a result, electric pulse signals proportional to the optical pulses are inputted to a trigger generating section 18. The section 18 discriminates the occurrence of the event from the inputted state of the optical pulses and outputs a trigger signal to each measurement circuit 11 to transmit the detecting data accumulated in the circuit 12 to the main body of this cosmic ray measuring instrument.