A power supply device for outputting voltage and current to a load, comprising: a current detection unit that detects current output to the load; a voltage detection unit that detects voltage output to the load; a current information reception unit that receives current information indicative of current output from a further power supply device to the load; a calculation unit that calculates control information for controlling voltage or current to be output to the load based on the current indicated by the current information received by the current information reception unit, the current detected by the current detection unit and the voltage detected by the voltage detection unit; a control unit that controls the voltage or current based on the control information calculated by the calculation unit; and being characterised by: a Z control information transmission unit that transmits the control information calculated by the calculation unit to the further power supply device, wherein if no voltage or no current is output to the load, information is transmitted and received by packet communication method, while if voltage or current is output to the load, information is transmitted and received by non-packet communication method for transmitting and receiving a predetermined amount of data, wherein non-packet communication method is in a direct communication mode during welding with high-speed communication, and wherein packet communication method is in a command communication mode during non-welding operation with no high-speed communication a current detection unit that detects current output to the load; a voltage detection unit that detects voltage output to the load; a current information reception unit that receives current information indicative of current output from a further power supply device to the load; a calculation unit that calculates control information for controlling voltage or current to be output to the load based on the current indicated by the current information received by the current information reception unit, the current detected by the current detection unit and the voltage detected by the voltage detection unit; a control unit that controls the voltage or current based on the control information calculated by the calculation unit; and being characterised by: a Z control information transmission unit that transmits the control information calculated by the calculation unit to the further power supply device, wherein if no voltage or no current is output to the load, information is transmitted and received by packet communication method, while if voltage or current is output to the load, information is transmitted and received by non-packet communication method for transmitting and receiving a predetermined amount of data, wherein non-packet communication method is in a direct communication mode during welding with high-speed communication, and wherein packet communication method is in a command communication mode during non-welding operation with no high-speed communication a Z control information transmission unit that transmits the control information calculated by the calculation unit to the further power supply device, wherein if no voltage or no current is output to the load, information is transmitted and received by packet communication method, while if voltage or current is output to the load, information is transmitted and received by non-packet communication method for transmitting and receiving a predetermined amount of data, wherein non-packet communication method is in a direct communication mode during welding with high-speed communication, and wherein packet communication method is in a command communication mode during non-welding operation with no high-speed communication