As represented by automated driving systems, the complexity of in-vehicle systems is rapidly increasing. The process related to the design, implementation, and validation of the system has become complicated. To reduce this complexity, a function description method suitable for the development of an automated driving system and an interpreter for directly executing this function description in software are proposed. To describe the function logically and strictly, the description sentence is based on an ontology model for traffic context. The description is also based on one-time predicate logic to enable verification by a theorem prover. By using this method, the behaviors of the automated driving system are directory linked with the description of the function definition.