To evaluate an exact bone mineral density, whena reference bar region in an X-ray picture of a mandible and a reference bar disposed beside the mandible is selected, the mean value and deviation of the brightness of this region are displayed by means of letter groups. The piston is reciprocably movable within the cylinder and is secured to the piston rod. A top compartment lid for closing the top compartment is pivotably supported on the carbody by a hinge device. Thus, the internal power supply voltages generated by the direct feedback voltage down-converting circuit and the voltage-dividing voltage down-converting circuit have no difference in temperature dependency from each other. The seat support preferably carries a seat having a planar laterally elongated base with terminal upstanding hip restraints to position the rider on the seat. The random holes can provide cladding for optical confinement when located around a fiber core.