A gantry drive system includes: a first motor configured to drive a driving object along a first axis; a second motor configured to drive the driving object along a second axis parallel with the first axis; and a motor control system configured to control the first and second motors. The motor control system includes a mode switch that performs a switching between a first control mode in which a position of the driving object on each of the first and second axes is individually controlled while reducing an inter-axis positional deviation between the first and second axes, and a second control mode in which a rotational state of the driving object is controlled while controlling a position of the driving object, based on detected positions of the driving object on the first and second axes.