Method of determining calibration curve and analysis method and apparatus using the same

One sigmoid calibration curve is split into three parts of low concentration region represented by a high degree function, intermediate concentration region represented by an exponential function and high concentration region represented by a high degree function according to the present invention. The boundary condition of the adjacent two functions is set so that the two functions have an equal slope at the boundary point; thereby, regression functions of the calibration curves in respective regions are found. The number of standard samples for finding a calibration curve can be reduced while the calibration curve found is of high accuracy.