Recent remarkable progress of energy and momentum resolutions in angle-resolved photoemission spectroscopy (ARPES) enables the direct observation of detailed electronic structure near the Fermi level, which is responsible for the occurrence of novel properties such as superconductivity. In this chapter, we report our recent high-resolution ARPES study on C6Ca, a graphite intercalation compound (GIC) with the highest superconducting transition temperature. The present ARPES study has established the existence of an interlayer band in GICs and its important contribu-tion to the superconductivity. Precise analysis of ARPES data on the superconducting gap and its symmetry has revealed the selective interaction of electrons with a specific bosonic mode possibly responsible for the superconductivity.