A PLC system includes a programmable logic controller including a PLC unit and a storing unit and an arithmetic-expression-data-creation supporting apparatus causing the PLC unit to create arithmetic expression data of an arithmetic expression to be executed by the PLC unit, and the arithmetic-expression-data-creation supporting apparatus includes an input unit to receive, from a user, the arithmetic expression including input information serving as a variable used in an arithmetic operation and a constant used in the operation and an arithmetic-expression processing unit to obtain a calculation sequence of the expression, and the storing unit retains an arithmetic-expression-data table for managing the arithmetic expression data including the input information, the constant and the calculation sequence, and the PLC unit includes an operation unit connected to the storing unit, to execute an arithmetic operation of the arithmetic expression according to the calculation sequence using the input information and the constant.