1. A quantitative analysis method for measuring a target element in a specimen, comprising: obtaining a spectrum generated from laser-induced plasma upon irradiation of a laser beam to a specimen including a target element; obtaining a fitting curve of a remaining element from the spectrum using Lorentzian function as a curve fitting method; obtaining a peak curve of the target element by subtracting the fitting curve of a remaining element from the spectrum; calculating a peak intensity of the peak curve of the target element and a peak intensity of the fitting curve of a remaining element; and obtaining a composition ratio of the target element based on a ratio of the peak intensity of the peak curve of the target element to the peak intensity of the fitting curve of the remaining element, wherein, when the correlation plot is represented by a straight line passing through the origin, the selected wavelengths are wavelengths at which a slope change is small with respect to laser energy change.