PURPOSE: To improve knocking detection precision by a method wherein the dimension, e.g., a diameter, depth, of a pressure introduction part is set so that the air circuit vibration frequency of the pressure introduction part which is opened to a combustion chamber and to which the pressure receiving part of a pressure-in-cylinder sensor is mounted is specified.

CONSTITUTION: A pressure-in-cylinder sensor 6 is situated to a cylinder head 2 of an engine in a manner that a pressure receiving part 6a is positioned facing a combustion chamber 1 through a pressure introduction part 7 formed in the shape of a cylinder. An output from the sensor 6 is inputted to a control unit 8 and performs control of combustion of an engine and prevents the occurrence of knocking. In this case, the dimension, e.g. a sectional area, depth, the volume of the pressure receiving part 6a, and an acoustic velocity, of the pressure introduction part 7 is set so that the air circuit vibration frequency of the pressure introduction part 7 is kept at a natural frequency being outside a resonance frequency area in a mode determined by the mounting position of the pressure- in-cylinder sensor 6. This constitution prevents an influence from being exercised on a resonance frequency area of knocking, and enables improvement of knocking detection precision.