The invention relates to an apparatus for preventing infection by malicious code, comprising: a database in which files installed in an agent system, DNA values for each part of the files, and index information for indicating whether each file is normal or malicious are stored; a calculation unit which calculates a DNA value for a part of a file for which an execution is requested in the agent system; and a file inspection unit which searches the database to extract, in a group, files having the DNA value calculated by the calculation unit, inspects whether an object file is normal or malicious on the basis of the index information on the files extracted in a group, and allows the execution of the object file or makes a request for the calculation of DNA values of other parts which selectively include one part of the object file.