A contents security apparatus for preventing Standard Definition (SD) contents which are protected targets of a level which is relatively lower than that of High Definition (HD) contents from being processed through a trust zone of a processor thereof and a method thereof are provided. The contents security apparatus includes a processor for operating a first Operating System (OS) and for storing authentication information of at least one or more contents and a second OS for limiting access to the first OS, wherein the first OS decrypts and processes contents with a high security level and wherein the second OS decrypts and processes contents with a low security level.