We developed composite electromagnetic wave absorbers made of fine aluminum particles dispersed in a polystyrene medium and evaluated their properties in order to realize metal-backed single-layer electromagnetic wave absorbers with a good absorption property in the GHz region. Both the magnetization and the magnetic loss caused by the eddy current flowing on the surface of the aluminum particles increased proportionally to the volume mixture ratio of the aluminum particles for all samples. The magnetization was almost independent of frequency but the magnetic loss decreased with increasing frequency for all samples. These results were in agreement with qualitative theoretical predictions.