There are provided a network attack detection apparatus and method capable of determining even unknown network attack, the apparatus connected between two networks or connected by port mirroring of an Ethernet switch to real-time monitor all packets flowing through the networks. The apparatus decodes a payload portion of an inputted network packet into a machine code instruction, determines whether an executable code is included in the decoded machine code by analyzing relationship between instructions, and determines whether the packet is harmful based on statistics with respect to a possibility that an executable code exists in a service and a certain transaction of the service when the executable code is included.