A device for calculating a flow rate (H) of an assist gas ejected from a tip end of a nozzle attached to a processing head in laser processing in which a laser beam (LB) guided from a laser oscillator to the processing head is applied on a workpiece (W) to perform desired processing, the assist gas flow rate calculation device comprising a control unit and a storage unit , the storage unit storing a first computer program calculating the flow rate (H) using a linear function (F1) for each nozzle having a pressure (P) of the assist gas in the processing head as a variable, and a second computer program calculating a slope (α) of the linear function (F1) using a function (F2) for each nozzle having a gap (G) from the tip end of the nozzle to a surface of the workpiece (W) as a variable, the control unit being configured to, when a known nozzle for which the linear function (F1) and the function (F2) are stored in the storage unit is used, acquire data indicating a value of the gap (G), calculate the slope (α) by substituting the value of the gap (G) into the function (F2) to execute the second computer program, acquire data showing a value of the pressure (P), and calculate the flow rate (H) by substituting the slope (α) and the value of the pressure (P) into the linear function (F1) to execute the first computer program, when an unknown nozzles for which the linear function (F1) and the function (F2) are not stored in the storage unit is used, calculate the flow rate (H) based on (a-1) a cross-sectional area of an opening of a known nozzle of a type same as a type of the unknown nozzle, among known nozzles for which the linear function (F1) and the function (F2) are stored in the storage unit , (b) a cross-sectional area of an opening of the unknown nozzle, and (c) the flow rate (H) when the known nozzle of the type same as the type of the unknown nozzle is used. the storage unit storing a first computer program calculating the flow rate (H) using a linear function (F1) for each nozzle having a pressure (P) of the assist gas in the processing head as a variable, and a second computer program calculating a slope (α) of the linear function (F1) using a function (F2) for each nozzle having a gap (G) from the tip end of the nozzle to a surface of the workpiece (W) as a variable, the control unit being configured to, when a known nozzle for which the linear function (F1) and the function (F2) are stored in the storage unit is used, acquire data indicating a value of the gap (G), calculate the slope (α) by substituting the value of the gap (G) into the function (F2) to execute the second computer program, acquire data showing a value of the pressure (P), and calculate the flow rate (H) by substituting the slope (α) and the value of the pressure (P) into the linear function (F1) to execute the first computer program, when an unknown nozzles for which the linear function (F1) and the function (F2) are not stored in the storage unit is used, calculate the flow rate (H) based on (a-1) a cross-sectional area of an opening of a known nozzle of a type same as a type of the unknown nozzle, among known nozzles for which the linear function (F1) and the function (F2) are stored in the storage unit , (b) a cross-sectional area of an opening of the unknown nozzle, and (c) the flow rate (H) when the known nozzle of the type same as the type of the unknown nozzle is used. <EOS>