Factory design support device and factory design support method

[Problem] To provide a plant design support device capable of suitably implementing the wiring design of a repeater without requiring a high degree of design skills. [Solution] This plant design support device (2) supports the wiring design of a plurality of repeaters that relay a primary cable connected to an instrument room provided in a plant and a plurality of secondary cables respectively connected to a plurality of devices provided in the plant. A plant design support device (2) is provided with: an information acquisition unit (201) that acquires, as plant design information, the installation position of an instrument room, the installation position of a device, and a primary cable installation area indicating an area in which a primary cable can be installed in a plant layout map, and that acquires repeater wiring design conditions relating to the wiring design of a repeater; and a determination unit (202) that determines, on the basis of the factory design information and the relay wiring design cond