1. An optical apparatus for acquiring structure information comprising: an optical branching device which splits a light outputted from a wavelength-swept light source into a sampling light and a reference light; a scanning device which scans a subject having a layer structure with the sampling light; and an signal processing device which acquires optical structure information of the subject by processing an interference signal between a return light which is reflected or backscattered at the subject and the reference light which has propagated a predetermined optical path length; wherein the signal processing device includes: a layer information extraction device which extracts layer information of the subject based on the interference signal; a feature value calculation device which calculates a feature value of the layer information; and an enhanced layer-structure image construction device which constructs an enhanced layer-structure image in which the layer structure is enhanced based on the feature value.