A knock detection device for an internal combustion engine make provisions to prevent erroneous decisions from being made due to a steplike signal change at the time of mask removal or to an overlap between an LC resonance generation period and a knock occurrence period in high engine rpm conditions. A low-frequency component is removed from an ion current signal by an HPF, and the resulting flat signal is supplied to a mask circuit, thereby reducing the amount of discontinuity occurring at the time of mask removal and suppressing noise in an output signal of a BPF to a level that does not affect knock detection. Further, by setting the gate period in a peak-hold circuit slightly delayed with respect to the mask removal timing, noise associated with the mask removal is completely eliminated. As mask signals used to specify the mask period, in addition to a mask signal for removing the mask when a predetermined crankshaft angle is reached, a second signal is provided for removing the mask when a prescribed time required for LC resonance noise to settle down has elapsed from the end of spark discharge, and the mask is removed only when both of the mask signals are deactivated.