Disclosed are a PLC system, and a method for controlling an input/output refresh period thereof. A PLC system according to an embodiment of the present disclosure comprises: a memory for storing an integrated operation program in which an input/output refresh run command is added and compiled through a program supporter; and a central processing unit which sequentially performs arithmetical processing of a plurality of control programs included in the integrated operation program, to control an operation of a plurality of expansion modules, wherein when arithmetical processing of at least one control program among the plurality of control programs has been completed, the central processing unit outputs processing result data of the at least one control program for which arithmetical processing has been completed, according to an input/output refresh run command which is additionally input.