To provide a method for measuring the amount of target nucleic acid in a sample, by accurately characterizing growth curves.

The method, by which a plurality of cycles of an amplification experiment are performed on a target nucleic acid and a standard nucleic acid, is provided with a step, in which signal data including a series of signal values indicating a quantity of the standard nucleic acid present during cycles of the standard amplification and including a series of signal values, indicating a quantity of the target nucleic acid present during cycles of the target amplification, are received; a step, in which a target growth curve value is defined using the target signal values, and a standard growth curve value is defined using the standard signal values; and a step, in which an initial amount of the target nucleic acid is calculated, according to a calibration equation which uses the initial amount of the standard nucleic acid, and the target and standard growth curve values, where the calibration equation is a nonlinear equation.