An endoscope apparatus includes an optical system including a focus lens, a connector to which an interchangeable optical system is connected, an image sensor configured to output a captured image based on the optical system and the interchangeable optical system, and a processor including hardware. The processor performs a determination process of determining whether the interchangeable optical system is a known or unknown optical system, implements a first step amount determination process when the interchangeable optical system is determined to be a known optical system, and implements a second step amount determination process when the interchangeable optical system is determined to be an unknown optical system and controls the focus lens based on step amount information determined.