The discovery of superconductivity in C 6 Yb and C 6 Ca (ref.1) has activated fierce debates on whether it is described within the conventional Bardeen-Cooper-Schrieffer scheme or some other exotic mechanisms are involved, because the superconducting transition temperature (Tc) is significantly higher than that of the alkali-metal graphite intercalation compounds intensively studied in the 1980s (refs2, 3, 4). The key to understand the mechanism of superconductivity lies in the superconducting energy gap associated with the formation of superconducting pairs. Here, we report the first direct observation of a superconducting gap in C 6 Ca by high-resolution angle-resolved photoemission spectroscopy. We found that the superconducting gap of 1.8-2.0 meV opens on the intercalant Fermi surface, whereas the gap is very small or absent on the Fermi surface derived from the * band of graphene layers. These experimental results unambiguously establish that the interlayer band has an essential role for the high-Tc superconductivity in C 6 Ca.