Decoding includes sensing a packet related to SSL handshake for connecting a SSL between a client and a server after a TCP session has been established between the client and the server in an SSL decoding device. If the packet for an SSL handshake is transmitted in a preset operating system, an SSL between the client and the SSL decoding device and an SSL between the SSL decoding device and the server is established. A TCP session between a virtual client corresponding to the client and a virtual server corresponding to the server is also established. A packet transmitted/received between the virtual client and the virtual server is transmitted when the TCP session is established. If a first SSL packet transferred from the client to the SSL decoding device is received, the SSL packet is decoded and transmitted to the security device and to the server.