In a knock sensor signal processing apparatus, a first apparatus which receives an engine knock sensor signal performs A/D conversions of the signal with a predetermined period and transmits successive pluralities of A/D values in parallel, by serial data communication, to a second apparatus in response to respective commands received from the second apparatus. The number of communication lines provided for transmitting the A/D values is made greater than those for transmitting commands from the second apparatus to the first apparatus, so that a high A/D conversion frequency together with low data rate of transmitting the A/D values can be achieved, thereby reducing communication-generated noise.
