This paper presents a novel exact order reduction approach for the Roesser (state-space) model of multidimensional systems using a common invariant subspace. It will be shown that this new exact order reduction approach can be applied even to those systems for which the existing approach based on common eigenvectors cannot do any further exact order reduction on them. As a consequence, the approach based on common eigenvectors can be viewed as a special case of the new proposed approach using a common invariant subspace.Examples will be also given to show the effectiveness as well as the details of the proposed approach.