Imaging device, endoscope apparatus, and imaging method

An imaging device includes an image sensor, an optical system forming an image of an object on the image sensor, and a processor. The optical system switches between a first state of capturing an image of the object with a single pupil and a second state of capturing an image of the object with two pupils. The processor generates a simulative phase difference image from a first captured image captured with the image sensor in the first state, and executes matching processing of comparing the simulative phase difference image with a second capture image captured with the image sensor in the second state to detect a phase difference between an image formed with one of the two pupils and an image formed with another one of the two pupils.