Although it is known that motor learning changes the corticospinal tract excitability, the time course of bilateral corticospinal tract excitability in the motor-learning process has not been clarified. The study aimed to investigate the time course of bilateral corticospinal tract excitability during the motor-learning process. Sixteen subjects performed 10 trials of the visuomotor tracking task by using their right index finger for one minute. The movement intensity of the visuomotor tracking task ranged from 5%-17% of the maximum index finger abduction force and the movement frequency was 0.5 Hz. To assess bilateral corticospinal excitability, we stimulated the bilateral primary motor cortex with transcranial magnetic stimulation between each trial and measured motor-evoked potential (MEP) from the bilateral first dorsal interosseous (FDI) muscle. Motor performance improved rapidly to the sixth trial (P < 0.05), and motor performance did not change from the seventh to the tenth trial. The MEP amplitude of the right FDI increased significantly from the fifth to the ninth trial relative to that before the task (P < 0.05). Conversely, the MEP amplitude of the left FDI did not change during the motor-learning process. This study revealed that primary motor cortex excitability on the contralateral side to the exercising muscle increased in the late motor learning stage and that primary motor cortex excitability on the ipsilateral side to the exercising muscle did not change in the motor-learning process.