To make it possible to stably move a mobile body even when the moving direction of the mobile body is changed in a straight mobile object driving apparatus for straight moving the mobile object such as a reading light source for scanning a radiation image recording medium.

A belt 21 spread by a pair of pulleys 22 is fixed on a linear motor 23 for moving the reading light source part 31 and a counter weight 26 is fixed on a part of the belt 21 which is opposed to the part fixed to the linear motor 23, so that the variation of driving load due to a change in the moving direction of the linear motor 23 is not generated. In addition, the counter weight 26 is provided with damping means (magnets 40, 41) for applying bias load to a direction rectangular to the spread direction of the belt 21 to suppress vibration which may be generated by the movement of the linear motor 23, so that the moving accuracy of the linear motor 23 is improved.