PURPOSE: To measure ferrous ion concentration directly, easily and simply at a job site by measuring respectively a pH value and an Eh value in the soil, and finding a concentration value from these measured values by means of operation according to a specific formula.

CONSTITUTION: Electric potential in the object soil A is detected by reference voltage (saturated silver chloride electrode) 12, and a reference electric potential value is set for measurement. A pH value is detected by a pH value detecting electrode (iridium oxide electrode) 13 according to this reference electric potential value. An Eh value is detected by an Eh value detecting electrode (platinum electrode) 14 according to a reference voltage value. Temperature correction is carried out on these pH value and Eh value by a temperature sensor 15. An electric potential difference measuring circuit 21 measures and outputs an electric potential difference corresponding to the pH value and the Eh value measured to the reference electric potential value. An operation circuit 22 carries out operation on ferrous ion concentration [Fe2+] from the obtained pH value and Eh value according to a formula [Fe2+]=4660-293pH-6.0. Provided that, [Fe2+]: wt. ppm and Eh: mVvsNHE are realized.