devices 3u and a plate like filter 4 arranged so as to cover a portion of the linear array above the human body 8 an x ray generator 5 such as a cathode ray tube is arranged and irradiates x ray beams generated via a k edge filter 24 to the human body 8 the k edge filter 24 is made of for instance nd and allows two kinds of x ray beams splitted to have high and low effective energies to pass therethrough for instance when a tube voltage of 80 kv is applied to the x ray generator 5 x ray beams having effective energies of 70 kev and 43 kev respectively are irradiated to the human body 8 through the k edge filter 24 each unit detection device 3u is comprised of a cdte radiation detector of pulse counting type having a predetermined small detection area an output therefrom is amplified by a preamplifier 25 is discriminated into signals of high and low levels by a pulse amplitude discriminator 26 and respective numbers of pulses of high and low levels discriminated are counted by a counter circuit 27 respective count numbers are outputted to a data processor 28 such as a microprocessor and the data processor 28 processes data inputted to obtain the bone salt as will be explained hereinbelow results obtained are displayed by a display 29 intensities i h0 and i l0 of the high energy and low energy x rays to be irradiated are measured beforehand by the image sensor 3 in.