1. An imaging apparatus comprising: an optical system; an imaging unit; an extractor that extracts a part of a first image as a reference image, the first image being received via the imaging unit; a first detector that obtains a first similar image from within the first image, the first similar image having a characteristic similar to a characteristic of the reference image; a calculator that obtains a positional relationship between the first similar image and the reference image; a second detector that obtains a second similar image from within a second image, the second image being received via the imaging unit later than the first image, the second similar image having a characteristic similar to the characteristic of the reference image; and a position detector that detects a position of a corresponding part of the second similar image in accordance with the positional relationship.