A method for cleaning a paint spray gun, which comprises a rotating atomization head to apply a coating material while rotating and an outer circumferential tube to cover the exterior of the rotating atomization head, has a cleaning solution application step for applying a cleaning solution to the external face of the outer circumferential tube, and a rotating atomization head rotation step for generating a rotational flow between the rotating atomization head and the outer circumferential tube by rotating the rotating atomization head. The cleaning solution flowing down the external face of the outer circumferential tube applied in the cleaning solution application step penetrates between the rotating atomization head and the outer circumferential tube by way of the rotational flow. Such method uses a small amount of cleaning solution for cleaning external and internal faces of the outer circumferential tube and the external face of a rotating atomization head.