In the semantic web context, there is a need to consider the nature of event entities in order to describe the semantic data of events in addition to that of objects and properties. This is because event data describes various features and behaviors of the occurrences of actions and changes in the real world. In this paper, we propose to establish an upper event ontology in order-sorted logic as an infrastructure for the semantic data of events. Our event ontology contains a classification of event entities (e.g., natural events and artificial events), event semantic functions (e.g., state change and existential change), and event relationships (e.g., causal relations and next-event relations).