This papers proposes Diagram for Ontology of Domain Terms and Tasks (referred to as $DODT^2$) in order to visualize Ontology. $DODT^2$ is divided into two diagrams. The first diagram is Diagram for Ontology of Domain Tasks (referred to as DODTasks). DODTasks visualize Ontology of Domain Tasks represented in Case Grammar. A component of this diagram describes a term which corresponds to each case of Case Grammar. In addition, to respond to any of various situations, three notations for DODTasks are proposed: Conditional Branch, Parallel Execution, and Random Order. The second diagram is Diagram for Ontology of Domain Terms (referred to as DODTerms). DODTerms represents the super-_{-relation of Domain Terms. Moreover, to assist Construction of Ontology, the author implemented $DODT^2$<} Editor. It can edit $DODT^2$. $DODT^2$ Editor involves functions to help construction of Ontology.