The present invention improves the readability of a ladder program. A controller (10) of a PC (1) displays program elements of a ladder program (51) and an electrical connection state thereof on a display (30), detects an instruction designating a first program element, determines whether the first program element is in an electrically connected state or an electrically disconnected state, specifies one or more causal elements according to whether the first program element is in an electrically connected state or an electrically disconnected state, and displays the specified one or more causal elements on the display (30) in a state discernible from other program elements.
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