A method of automatically setting a protocol, performed in a server communicating with clients in a Programmable Logic Controller system, wherein the server comprises a memory area, referred to as a protocol stack, in which a plurality of protocols are stored, and a stack register designating a level in the protocol stack; the method comprising: receiving a request frame from a client; analyzing a protocol of the request frame based on a protocol stack, the analyzing comprising: reading (S720, S910) out a protocol stored the protocol stack, at the level designated by the stack register; determining (S730, S920) whether the read protocol coincides with the protocol of the request frame; and transmitting (S740, S921) a response frame to the client for the request frame using the read protocol when the read protocol coincides with the protocol of the request frame; wherein the protocol stack refers to a memory area in which a plurality of protocols are stored, the reading out of a protocol initially occurs with the stack register designating the highest level at the protocol stack, and when the read protocol does not coincide with the protocol of the request frame, the analyzing further comprises: determining whether the protocol of the request frame exists in a memory area of the protocol stack, by changing the stack register to indicate a memory area of a lower level than the memory area of the read protocol, and by repeating the analyzing to determine the response frame for the request frame, or when the protocol does not exist at in the protocol stack, the analysing further comprises: determining whether the protocol of the request frame has been stored in a protocol DB, wherein the determining whether the protocol of the request frame is stored in the protocol DB comprises: storing the protocol of the request frame in the protocol DB if the protocol of the request frame is not stored in the protocol DB; and increasing a request number of the protocol of the request frame if the protocol of the request frame is stored in the protocol DB, wherein the increasing the request number comprises providing alarm notifying a user of a current request status of the protocol of the request frame if the request number is above a predetermined number. receiving a request frame from a client; analyzing a protocol of the request frame based on a protocol stack, the analyzing comprising: reading (S720, S910) out a protocol stored the protocol stack, at the level designated by the stack register; determining (S730, S920) whether the read protocol coincides with the protocol of the request frame; and transmitting (S740, S921) a response frame to the client for the request frame using the read protocol when the read protocol coincides with the protocol of the request frame; reading (S720, S910) out a protocol stored the protocol stack, at the level designated by the stack register; determining (S730, S920) whether the read protocol coincides with the protocol of the request frame; and transmitting (S740, S921) a response frame to the client for the request frame using the read protocol when the read protocol coincides with the protocol of the request frame; wherein the protocol stack refers to a memory area in which a plurality of protocols are stored, the reading out of a protocol initially occurs with the stack register designating the highest level at the protocol stack, and when the read protocol does not coincide with the protocol of the request frame, the analyzing further comprises: determining whether the protocol of the request frame exists in a memory area of the protocol stack, by changing the stack register to indicate a memory area of a lower level than the memory area of the read protocol, and by repeating the analyzing to determine the response frame for the request frame, or when the protocol does not exist at in the protocol stack, the analysing further comprises: determining whether the protocol of the request frame has been stored in a protocol DB, wherein the determining whether the protocol of the request frame is stored in the protocol DB comprises: storing the protocol of the request frame in the protocol DB if the protocol of the request frame is not stored in the protocol DB; and increasing a request number of the protocol of the request frame if the protocol of the request frame is stored in the protocol DB, wherein the increasing the request number comprises providing alarm notifying a user of a current request status of the protocol of the request frame if the request number is above a predetermined number. determining whether the protocol of the request frame has been stored in a protocol DB, wherein the determining whether the protocol of the request frame is stored in the protocol DB comprises: storing the protocol of the request frame in the protocol DB if the protocol of the request frame is not stored in the protocol DB; and increasing a request number of the protocol of the request frame if the protocol of the request frame is stored in the protocol DB, storing the protocol of the request frame in the protocol DB if the protocol of the request frame is not stored in the protocol DB; and increasing a request number of the protocol of the request frame if the protocol of the request frame is stored in the protocol DB, wherein the increasing the request number comprises providing alarm notifying a user of a current request status of the protocol of the request frame if the request number is above a predetermined number.