A control device includes a casing and a control circuit substrate arranged inside the casing. The control circuit substrate includes: a control circuit mounted on the control circuit substrate; an internal connector connectable to a communication cable inside the casing, the communication cable being connected to an expansion circuit substrate; and an external connector connectable to another communication cable outside the casing, the another communication cable being connected to another expansion circuit substrate. The control circuit is connected to the internal connector and the external connector in parallel.