An adapter optical system, which is an optical system configured to guide reflection light reflected by an object and passing through an imaging lens to a liquid resonant lens, includes: a first lens unit removably disposed at a position where the reflection light passing through the imaging lens enters and constituting an image-side telecentric optical system; and a second lens unit disposed at a position where the reflection light passing through the first lens unit enters and constituting a finite correction optical system in combination with the liquid resonant lens.