This invention provides a rubber composition and an electromagnetic wave absorbing sheet that can realize excellent electromagnetic wave absorbance properties while only having a small thickness and is also excellent in flexibility. The rubber composition comprises 100 parts by weight of a rubber component such as natural rubber and 300 to 1500 parts by weight, preferably 500 to 1000 parts by weight, of carbonyl iron. The incorporation of a predetermined amount of carbonyl iron in the rubber component can realize excellent electromagnetic wave absorbance properties, whereby the thickness of the electromagnetic wave absorbing sheet for providing a necessary electromagnetic wave absorbance amount can be reduced. The rubber composition comprising carbonyl iron incorporated together with the rubber component has low hardness, and is also excellent in flexibility. Accordingly, the rubber composition has good processability, is less likely to break, and is also excellent in durability. The rubber component may contain not less than 5 parts by weight of a weather-resistant rubber in 100 parts by weight of the rubber component and further may contain 1 to 10 parts by weight of carbon black.