the digital signal processing means 16 by a closed gain in the periods other than the knock detection period closed periods cls in addition a predetermined interpolation period int is provided between the open period opn and each of the closed periods cls and in the interpolation period int the processing result by the digital signal processing means 16 is multiplied by a linear interpolation value between the open gain and a predetermine interpolation gain therefore since a sudden change in the knock signal level does not occur even when various kinds of noise come in and out of the knock detection period erroneous knock detection due to the knock determination threshold value slowly following the change can be suppressed and in addition since the knock detection period is not unnecessarily expanded an excellent effect can be expected in which erroneous knock detection outside the knock detection period can be avoided and as a result knock controllability can be enhanced moreover although explanations have been made on the interpolation period at the back end of the knock detection period in each embodiment described above the same configuration can be applied to the front end of the knock detection period thereby bringing about the same effect moreover in a state in which different noise is superimposed before and after the knock detection period the.