We study Ruelle-Perron-Frobenius operators for locally expanding and mixing
dynamical systems on general compact metric spaces associated with potenti
als satisfying the Dini condition. In this paper, we give a proof of the Ru
elle Theorem on Gibbs measures. It is the first part of our research on the
subject. The rate of convergence of powers of the operator will be present
ed in a forthcoming paper.