The present invention provides an aluminum-diamond composite which combines high thermal conductivity and a coefficient of thermal expansion close to a semiconductor element, and in which the difference between the thicknesses of both surfaces is reduced so as to be suitable for use as a heat sink etc. for a semiconductor element. Provided is a flat plate-shaped aluminum-diamond composite that has an aluminum-diamond composite part and a surface layer that coats both surfaces of the composite part and includes a metal that has aluminum as a principal component, wherein: the composite part is composed of a composite material that is composed of an aluminum or aluminum alloy matrix and diamond particles dispersed in said matrix; the composite material is composed of a diamond powder in which diamond particles having a particle size of 1-20 μm, inclusive, make up 10-40 vol % of the diamond particles and diamond particles having a particle size of 100-250 μm, inclusive, make up 50-80 vol %, said powder not containing diamond particles having a particle size of less than 1 μm or diamond particles having a particle size of more than 250 μm; and the average value for the differences in in-plane thickness per 50 mm×50 mm is 100 μm or less.