A logic diagram processing device has a logic diagram relocating unit which relocates operation elements while maintaining control logics equivalently so that the operation elements as compared overlap in the case where a plurality of designated logic diagrams are overlapped, and a logic diagram overlapping unit which overlaps the plurality of designated logic diagrams in which the operation elements are relocated by the logic diagram relocating unit. The logic diagram processing device also includes a display unit which displays the plurality of designated logic diagrams overlapped by the logic diagram overlapping unit in a predetermined display method by which a common part or the difference of the operation elements can be discriminated, and a display control unit which switches the predetermined display method to display in the display unit in accordance with an instruction from the outside.