PROBLEM TO BE SOLVED: To provide a method and a system which are capable of measuring moisture content contained in a ground material continuously and accurately.SOLUTION: A sample T of a ground material N, in which various grain diameters exist in a mixed manner, is divided into a plurality of constituent materials T1,T2,... Tn by a predetermined grain diameter range. A proportional parameter Pi (ai,bi,ci,di for example) between reflectance or transmissivity Si of predetermined wavelength λi when each of the constituent materials Ti is irradiated with near infrared light and water volume Wi of the constituent materials Ti is detected, and a synthesized parameter Q(&Sgr;(Ii ai),&Sgr;(Ii bi),&Sgr;(Ii ci),&Sgr;(Ii di) for example) in which the proportional parameter Pi is weighted corresponding to a grain diameter range Ii by each constituent material Ti is created. Based on the reflectance or the transmissivity Si of the predetermined wavelength λi when the ground material N of an object to be me
