There is provided a cooling structure whose performance for cooling ring members and brushes is enhanced. In a cooling structure applied to a slip ring device including ring members provided to an input shaft and brushes contacting with these ring members, the input shaft includes a shaft member where an external spline portion is formed, and a cylindrical member that is installed over the external circumference of the shaft member so that an internal spline portion formed on the cylindrical member is meshed with the external spline portion, and that the ring members are fixed to the cylindrical member. The internal spline portion is formed upon a portion of the inner circumferential surface of the cylindrical member that lies on the radially inward side of the ring members.