detected pressure waveform produced in the case wherein such a phenomenon occurs is shown in fig5 a and that of the maximum pressure value which is outputted is indicated in fig5 b in the case of an ignition failure the resulting maximum pressure value does not reach the lower boundary or delimitation value p2 u03b4p of the reference value range whereby this abnormal phenomenon is detected and at the same time the closure of the shut off valve 16 and the operation of the alarm 17 are carried out as described hereinabove on the other hand in the case where an abnormality occurs in the gas injection valve 19 and a large quantity of gas flows into the cylinder 1 the value of the maximum pressure within the cylinder rises remarkably as indicated by curve 22 to 23b in fig4 consequently conversely to the above described abnormal phenomenon the maximum pressure value exceeds the upper boundary value p1 u03b4p whereby it is possible to detect the occurrence of the abnormality the above described reference value range of the maximum pressure in the cylinder fluctuates with the operational conditions of the engine that is the load and the revolution speed accordingly by drafting beforehand a zonal map of normal pressure ranges corresponding to these variations as indicated in fig6 storing this map in a memory component of the system and providing beforehand means for computing the normal pressure range at any time in the operation in response to signals indicative of the operational conditions at that time it becomes possible to obtain accurate and.