A variable focal length optical system includes: an imaging lens configured to condense a parallel beam of light from an objective lens to form an intermediate image; a relay system including, in sequence from an object side, a first relay lens and a second relay lens and configured to relay the intermediate image to infinity; and a lens system of a liquid resonant type which is located between the first relay lens and the second relay lens. A front principal point of the lens system is located to be conjugated with an exit pupil of the objective lens and an expression, 1.90≤fTube/fR1≤2.15, is satisfied. In the expression, fTube denotes a focal length of the imaging lens and fR1 denote a focal length of the first relay lens.