To provide a method for providing a further precise estimation result when requiring the inspection of proportion of a hardened concrete.

This method of estimating composition of hardened concrete comprises estimating the cement proportion ratio in the hardened concrete of subject from the CaO content (%) of an absolute dry fine powdery analyzing sample, further successively estimating the unit cement quantity (kg/m3) in the hardened concrete, the coupling water component ratio in the hardened concrete, the absolute dry aggregate component ratio in the hardened concrete, the absolute dry unit aggregate quantity (kg/m3) in the hardened concrete, the unit cement volume (l/m3) in the hardened concrete, and subtracting an estimated apparent air volume (l/m3) to estimate the unit water quantity (l/m3) in the hardened concrete.