In situ angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS) have been used to study the electronic structure of Pb thin films grown on a Si(111) substrates. The experiments reveal that the electronic structure near the Fermi energy is dominated by a set of m-shaped subbands because of strong quantum confinement in the films, and the tops of the m-shaped subbands form an intriguing ring-like Van Hove singularity. Combined with theoretical calculations, we show that it is the Van Hove singularity that leads to an extremely high density of states near the Fermi energy and the recently reported strong oscillations (with a period of two monolayers) in various properties of Pb films.