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; 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 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 peak curve of the remaining element.