To obtain a brake disk composed of aluminum matrix composite, being lightweight and having wear resistance, wear characteristic, friction coefficient, and brake retentivity, by dispersing aluminum borate whiskers having Si3N4 coating layer in an aluminum or aluminum-alloy matrix.

A formed part of prescribed shape, composed of aluminum borate whiskers having Si3N4 coating layer, preferably, of 0.05 to 0.4 μm thickness, is set in a mold of a pressure casting machine, and molten aluminum or aluminum alloy is poured into the mold and pressure casting is carried out, by which the brake disk composed of aluminum matrix composite of 12 to 27% volume filling rate of whisker is obtained. It is preferable to use, as the aluminum alloy for the above matrix, an alloy which has a composition consisting of, by weight, 1-14% Si, 0.1-5.5% Cu, 0.1-0.9% Mg, and the balance Al with inevitable impurities and containing, if necessary, ≤1.0% Fe, ≤0.3% Mn, ≤0.25% Ti, ≤1.3% Ni, and ≤0.3% Zn.