A device for detection of knocking in a diesel engine has a pressure sensor including a piezoelectric element and detecting a rate of change in combustion pressure, an angle sensor for detecting the rotation of the engine or engine speed, and a knocking detector for receiving outputs from the pressure and angle sensors. The knocking detector serves to detect a rate of change in combustion pressure per unit angle upon necessary correction by the engine speed and to compare the obtained rate with a reference rate of 5 atm/deg, at which knocking occurs. Knocking is detected when the detected rate exceeds the reference rate.