Abstract: A composite member has a substrate made of a composite material having SiC combined with magnesium or a magnesium alloy, and has a warpage degree of not less than 0.01×10?3 and not more than 10×10?3, the warpage degree being defined as lmax/Dmax, where lmax being a difference between a maximum value and a minimum value of surface displacement of one surface of composite member measured along a longest side thereof, and Dmax being a length of the longest side. Thereby, a composite member capable of efficiently dissipating heat to an installation object, a heat-dissipating member using the composite member, and a semiconductor device having the heat-dissipating member are provided.