1. A simulation apparatus for testing a programmable logic controller (PLC) code that is input to and output from a PLC for operation of the PLC, the simulation apparatus comprising: a PLC code storing unit configured to store the PLC code comprising a plurality of PLC symbols, wherein each of the plurality of PLC symbols comprises a plurality of levels for generating an input/output model, wherein the plurality of levels are identified by an identifier; a model defining unit configured to define facilities that communicate with the PLC as input/output models using the plurality of PLC symbols; a symbol extracting unit configured to extract PLC symbols which are input and output between the PLC and the input/output models from the PLC code storing unit; a state defining unit configured to analyze the extracted PLC symbols to define operation states of the defined facilities for the input/output models; and a state control unit configured to control the conversion of the operation states by implementing a transition event for operation state transition, an entering event for entering an operation state, and a leaving event for leaving a current operation state with respect to each of the operation states in response to the extracted PLC symbols.