A device for calculating a flow rate 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 guided from a laser oscillator to the processing head is applied on a workpiece to perform desired processing, the assist gas flow rate calculation device comprising a control unit and a storage unit (60, 70), the storage unit (60, 70) storing a first computer program calculating the flow rate using a linear function for each nozzle having a pressure of the assist gas in the processing head as a variable, and a second computer program calculating a slope (α) of the linear function using a function for each nozzle having a gap from the tip end of the nozzle to a surface of the workpiece as a variable, the control unit being configured to, when a known nozzle for which the linear function and the function are stored in the storage unit (60, 70) is used, acquire data indicating a value of the gap, calculate the slope (α) by substituting the value of the gap into the function to execute the second computer program, acquire data showing a value of the pressure, and calculate the flow rate by substituting the slope (α) and the value of the pressure into the linear function to execute the first computer program, when an unknown nozzles for which the linear function and the function are not stored in the storage unit (60, 70) is used, calculate the flow rate 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 and the function are stored in the storage unit (60, 70), (b) a cross-sectional area of an opening of the unknown nozzle, and (c) the flow rate when the known nozzle of the type same as the type of the unknown nozzle is used. the storage unit (60, 70) storing a first computer program calculating the flow rate using a linear function for each nozzle having a pressure of the assist gas in the processing head as a variable, and a second computer program calculating a slope (α) of the linear function using a function for each nozzle having a gap from the tip end of the nozzle to a surface of the workpiece as a variable, the control unit being configured to, when a known nozzle for which the linear function and the function are stored in the storage unit (60, 70) is used, acquire data indicating a value of the gap, calculate the slope (α) by substituting the value of the gap into the function to execute the second computer program, acquire data showing a value of the pressure, and calculate the flow rate by substituting the slope (α) and the value of the pressure into the linear function to execute the first computer program, when an unknown nozzles for which the linear function and the function are not stored in the storage unit (60, 70) is used, calculate the flow rate 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 and the function are stored in the storage unit (60, 70), (b) a cross-sectional area of an opening of the unknown nozzle, and (c) the flow rate when the known nozzle of the type same as the type of the unknown nozzle is used.