To select a channel adapted to detection of the position of a liver. The position “m” of the border between the liver and the lung is obtained from a profile. A sum Sliver of signal intensities in a liver region R1 and a sum Slung of signal intensities in a lung region R2 are calculated. After obtaining the sums Sliver and Slung of the signal intensities, Sliver and Slung are compared to determine whether Sliver is equal to or less than Slung. In the case where Sliver is equal to or less than Slung (Sliver≦Slung), a channel is not selected as a channel used at the time of detecting the position of the edge of the liver. On the other hand, in the case where Sliver is larger than Slung (Sliver>Slung), a channel is selected as a channel used at the time of detecting the position of the edge of the liver.
