A welding system comprising: an input/output device having a function of inputting and outputting information to an external device; and a welding apparatus arranged to communicate with the input/output device, wherein the input/output device includes: a general-purpose input circuit section arranged to input information from the external device; a general-purpose output circuit section arranged to output information to the external device; and an input/output control section arranged to communicate with at least one of the general-purpose input circuit section, the general-purpose output circuit section, and the welding apparatus, wherein the welding apparatus includes: an input/output assignment storing section having a first memory region arranged to store an assignment state of an input channel of the general-purpose input circuit section to an input function of the welding apparatus, and further arranged to store an assignment state of an output channel of the general-purpose output circuit section to an output function of the welding apparatus; and a welding apparatus-side input/output control section arranged to operate an input function of the welding apparatus based on an input signal transmitted from the input/output control section, or transmitting, to the input/output control section, an output signal indicating which channel of the general-purpose output circuit section an output function of the welding apparatus is assigned to, wherein at least one of the input/output control section and the welding apparatus-side input/output control section is arranged to communicate with an external controller (19, 20), wherein the welding system is arranged to store data related to the assignment in the input/output assignment storing section using the external controller (19, 20), and is further arranged to alter data related to the assignment stored in the input/output assignment storing section using the external controller (19, 20). an input/output device having a function of inputting and outputting information to an external device; and a welding apparatus arranged to communicate with the input/output device, wherein the input/output device includes: a general-purpose input circuit section arranged to input information from the external device; a general-purpose output circuit section arranged to output information to the external device; and an input/output control section arranged to communicate with at least one of the general-purpose input circuit section, the general-purpose output circuit section, and the welding apparatus, a general-purpose input circuit section arranged to input information from the external device; a general-purpose output circuit section arranged to output information to the external device; and an input/output control section arranged to communicate with at least one of the general-purpose input circuit section, the general-purpose output circuit section, and the welding apparatus, wherein the welding apparatus includes: an input/output assignment storing section having a first memory region arranged to store an assignment state of an input channel of the general-purpose input circuit section to an input function of the welding apparatus, and further arranged to store an assignment state of an output channel of the general-purpose output circuit section to an output function of the welding apparatus; and a welding apparatus-side input/output control section arranged to operate an input function of the welding apparatus based on an input signal transmitted from the input/output control section, or transmitting, to the input/output control section, an output signal indicating which channel of the general-purpose output circuit section an output function of the welding apparatus is assigned to, an input/output assignment storing section having a first memory region arranged to store an assignment state of an input channel of the general-purpose input circuit section to an input function of the welding apparatus, and further arranged to store an assignment state of an output channel of the general-purpose output circuit section to an output function of the welding apparatus; and a welding apparatus-side input/output control section arranged to operate an input function of the welding apparatus based on an input signal transmitted from the input/output control section, or transmitting, to the input/output control section, an output signal indicating which channel of the general-purpose output circuit section an output function of the welding apparatus is assigned to, wherein at least one of the input/output control section and the welding apparatus-side input/output control section is arranged to communicate with an external controller (19, 20), wherein the welding system is arranged to store data related to the assignment in the input/output assignment storing section using the external controller (19, 20), and is further arranged to alter data related to the assignment stored in the input/output assignment storing section using the external controller (19, 20).