A ventilated rotor to be disclosed has a first slide board, a second slide board which faces the first slide board and N-numbered (N: natural number, not less than 2) ribs provided between the first and second slide boards. The first and second slide boards have a common central axis, and the N-numbered ribs include ribs, which are in reference providing positions separated from each other at equal angular intervals in the circumferential direction and are elongated in a radial direction perpendicular to the central axis, and N/n-numbered (n: natural number, not less than 2, N/n: natural number) ribs, which are shifted from the reference providing positions by a prescribed angle in one direction of the circumferential direction and are elongated in the radial direction. In another way, the N-numbered ribs includes N/n-numbered ribs which have divided portions divided with separation from each other in the radial direction. Moreover, the N/n-numbered ribs are respectively placed first in every n-numbered ribs of the N-numbered ribs.