PURPOSE: To permit the correct detection for the top signal as the standard of the control timing of each control mechanism for an engine by removing the noise mixed during the above-described period by cutting-off the signals in a top signal line.

CONSTITUTION: During engine revolution, the revolution-angle signal Sa supplied from a revolution-angle sensor 2 installed onto a ring gear 7 on a crankshaft 6 is input into a reading-out indication counter 8 in a noise preventing circuit 3. The top signal Sb supplied from a top sensor 1 installed onto one tooth 5 on a cam shaft 4 is input into an AND gate 9. Said AND gate 9 receives a reading- out indication signal Sc and outputs a normalized top signal Sd from which noise is removed. When the signal Sd is generated, counters 8 and 10 are reset. When the counter 8 is reset, the revolution angle signal Sa is counted by a certain number, and the reading-out indication signal Sc is outputted. When the signal Sc is not received, namely during reading-out prohibiting period, the gate 9 is cut-off.