The purpose of this paper is to construct certain diffusion processes, in particular a Brownian motion, on a suitable kind of infinite-dimensional manifold. This manifold is a Banach manifold modelled on an abstract Wiener space. Roughly speaking, each tangent space T x {T_x} is equipped with a norm and a densely defined inner product g ( x ) g(x) . Local diffusions are constructed first by solving stochastic differential equations. Then these local diffusions are pieced together in a certain way to get a global diffusion. The Brownian motion is completely determined by g g and its transition probabilities are proved to be invariant under d g {d_g} -isometries. Here d g {d_g} is the almost-metric (in the sense that two points may have infinite distance) associated with g g . The generalized Beltrami-Laplace operator is defined by means of the Brownian motion and will shed light on the study of potential theory over such a manifold.