As a generalization of fill-in free property of a sparse positive definite real symmetric matrix with respect to the Cholesky decomposition, we introduce a notion of (quasi-)Cholesky structure for a real vector space of symmetric matrices. The cone of positive definite symmetric matrices in a vector space with a quasi-Cholesky structure admits explicit calculations and rich analysis similar to the ones for Gaussian selsction model associated to a decomposable graph. In particular, we can apply our method to a decomposable graphical model with a vertex pemutation symmetry.[-]