The infinite-layer compound ACuO 2 (A=alkaline-earth ions) is regarded as the most suitable material for exploring the fundamental nature of the CuO 2 plane because it does not contain a charge-reservoir block, such as a rock-salt or a fluorite like block. We report that superconductivity in the infinite-layer compound Sr 0 . 9 La 0 . 1 CuO 2 is of a three-dimensional nature, in contrast to the quasi two-dimensional superconducting behavior of all other cuprates. The key observation is that the c-axis coherence length is longer than the c-axis lattice constant even at zero temperature. This means that the superconducting order parameter of one CuO 2 plane overlaps with those of neighboring CuO 2 planes all the temperatures below T c .