A plant design assistance method of assisting in, through use of a computer, wiring design of a plurality of relay devices serving as a relay between a primary cable to be connected to a control room installed in a plant and a plurality of secondary cables to be connected to a plurality of devices, respectively, the plurality of devices being installed in the plant, the plant design assistance method comprising:
- an information acquisition step of acquiring, as plant design information, an installation position of the control room, an installation position of each of the plurality of devices, and a primary cable installation region indicating a region in which the primary cable is installable, in a layout diagram of the plant, and acquiring a relay device wiring design condition regarding the wiring design of the plurality of relay devices;
- and
- a determination step of determining an installation position of a relay device in the layout diagram and allocation of a device connected to the relay device, based on the plant design information and the relay device wiring design condition acquired in the information acquisition step.