The present invention ensures that the warping of an annular disk body of a brake disk, in the direction of a rotational axis, due to a friction heat, is inhibited to a large extent. The brake disk includes an aluminum alloy matrix, a plurality of alumina aggregates dispersed in the matrix, and a plurality of fine alumina grains. The alumina aggregates have an aspect ratio A larger than 1. In an annular disk body, the alumina aggregates are oriented with their lengthwise directions aligned with the radial direction of the disk body.