1. A malicious-process-determining method for determining whether or not a malicious process is executed based on instruction codes included in received data, using a data processing apparatus comprising receiving means for receiving data including a plurality of instruction codes having different byte lengths and storing means for storing received data, characterized by comprising the steps of: reading data stored in the storing means sequentially by one byte; detecting a pattern of occurrence of instruction codes that are contained in each of data sequences, which are respectively composed of a contiguous sequence of byte strings and have a first byte corresponding to a different read address; and determining whether a process, which is executed in each of data sequences, is a malicious process or not based on the detected pattern of occurrence of instruction codes.