PURPOSE: To accurately detect the occurrance of knock with simple constitution by making a vibrational state of a reference cylinder a reference level and detecting a vibration having larger amplitude than that of the reference level with a sensor fitted to another bank to which the reference cylinder does not belong.

CONSTITUTION: Pulser coils 17AW17C are fixed around a permanent magnet 16 of a crankshaft rotor of a V type 6-cylindered engine and pulses P1 and P4 of the 1st and the 4th cylinders are caused on the coil 17 for every rotation by 60° of the crankshaft. The 2nd cylinder 11B is made the reference cylinder and the sensor 13 is fitted to the right bank 10R to which the reference cylinder does not belong and a detected value of the sensor 13 is integrated between a pulse P2 of the 2nd cylinder and a pulse P3 following it. Accordingly, the vibrational state without being influenced by the knock is made the reference vibration level and the knock taking place at another time is compared with it and detected, by which the occurrence of knock can be certainly detected with simple constitution.