The paper proposes an approach to information retrieval based on the use of a structure (ontology) both for document (resp. query) indexing and query evaluating. The conceptual structure is hierarchical and it encodes the knowledge of the topical domain of the considered documents. It is formally represented as a tree. In this approach, the query evaluation is based on the comparison of minimal sub-trees containing the two sets of nodes corresponding to the concepts expressed in the document and the query respectively. The comparison is based on the computation of a degree of inclusion of the query tree in the document tree. Experiments undertaken on MuchMore benchmark showed the effectiveness of the approach. Keywords Information retrieval, concept-based retrieval, query/document completion driven by ontology.