PURPOSE: To have precision sensing of the fuel injection timing at all times by furnishing a window near the expected fuel injection timing, and thereby sensing only real timing of the start of fuel injection.

CONSTITUTION: A signal S1 from a vibration sensor 1 is given to a preprocessing circuit 2. An injection start sensing circuit 5 makes a reference value SL1 to be used for sensing the start of fuel injection. Each rise of No.3 pulse signal PE indicates that fuel injection in each cylinder has terminated. A window generation circuit 12 generates a window signal in accordance with the specific period from before fuel injection till immediately after finishing the fuel injection. An AND circuit 15 carries out an operation for the logical product of No.2 pulse signal from a comparator circuit 10 and a window signal PW' from said window generator circuit 12, which is output as the injection start signal PS. Thus precise fuel injection timing can be sensed at all times.