An apparatus for detecting an in-vehicle network attack, is configured to cumulatively count packets for each device that has a respective ID and is connected to an in-vehicle network bus. The apparatus is configured to cumulate a check value every time the packets are cumulatively counted to calculate a cumulated value, and determine that an attack is conducted when an average cumulated value obtained by dividing the cumulated value by a cumulative counted value does not exceed a first threshold value.