A method of manufacturing a programmable logical controller (100, 100A), the method comprising: providing a programmable logical controller including a substrate, the substrate including an interface unit to which an external device (200, 200A) having input and output terminals (P11, P12, P13) is attachable and a first storage unit, the interface unit including a processor circuit, a first connector terminal and a second connector terminal, wherein each of the first and second connector terminals (P1, P2) is configured to be connectable to one of the input and output terminals (P11, P12) of the external device (200, 200A); wherein the method is characterized in further comprising: forming a predetermined connection by connecting two of the input and output terminals (Pll, P12) of the external device (200, 200A) to the first and second connector terminals (P1, P2) respectively, wherein the first and second connector terminals (P1, P2) are short-circuited with each other when the predetermined connection is formed; determining the formation of the predetermined connection by comparing a voltage received at the first connector terminal and a voltage received at the second connector terminal with a predetermined short-circuit voltage; causing the processor circuit to load a system starting data of the programmable logical controller (100, 100A) from a second storage unit of the external device (200, 200A) in which the system starting data is stored and store the read system starting data to the first storage unit through a third connector terminal connecting the programmable logical controller (100, 100A) to the second storage unit of the external device (200, 200A) when it is determined that the predetermined connection is formed; and causing the processor circuit to load the system starting data of the programmable logical controller (100, 100A) stored in the first storage unit and store the read system starting data in a main storage unit when it is determined that the predetermined connection between the first and second connector terminals (P1,P2) is not formed. providing a programmable logical controller including a substrate, the substrate including an interface unit to which an external device (200, 200A) having input and output terminals (P11, P12, P13) is attachable and a first storage unit, the interface unit including a processor circuit, a first connector terminal and a second connector terminal, wherein each of the first and second connector terminals (P1, P2) is configured to be connectable to one of the input and output terminals (P11, P12) of the external device (200, 200A); wherein the method is characterized in further comprising: forming a predetermined connection by connecting two of the input and output terminals (Pll, P12) of the external device (200, 200A) to the first and second connector terminals (P1, P2) respectively, wherein the first and second connector terminals (P1, P2) are short-circuited with each other when the predetermined connection is formed; determining the formation of the predetermined connection by comparing a voltage received at the first connector terminal and a voltage received at the second connector terminal with a predetermined short-circuit voltage; causing the processor circuit to load a system starting data of the programmable logical controller (100, 100A) from a second storage unit of the external device (200, 200A) in which the system starting data is stored and store the read system starting data to the first storage unit through a third connector terminal connecting the programmable logical controller (100, 100A) to the second storage unit of the external device (200, 200A) when it is determined that the predetermined connection is formed; and causing the processor circuit to load the system starting data of the programmable logical controller (100, 100A) stored in the first storage unit and store the read system starting data in a main storage unit when it is determined that the predetermined connection between the first and second connector terminals (P1,P2) is not formed.