A low-coherence light is split into a signal light and a reference light. The optical path length of the reference light is switched to optical path lengths that correspond to a first depth zone and a second depth zone. When forming a tomographic image of the second depth zone, in an optical system that condenses the signal light to the first depth zone when a measured object and an objective lens are located at a predetermined working distance, while being positioned at the working distance, a depth zone switching lens that transitions the depth at which the signal light is condensed to the second depth zone is inserted.