PURPOSE: To improve the accuracy of the osteosalt quantity to be measured by correcting beam hardening of X-rays in the osteosalt measuring instrument by a double energy X-ray absorption method.

CONSTITUTION: This instrument is provided with means 13 of expressing the logarithm of the X-ray transmission ratio of X-rays of two kinds of energies, high and low, by the sum of the product of the component quantity of a body 5 to be measured and absorption coefft. thereof and the constant term, calculating the absorption coeffts. of the respective components as the function of the logarithm of the X-ray transmission ratio and calculating the osteosalt quantity from the equation thereof.