1. An optical coherence tomography comprising: a light source configured to change a wavelength of light to be outputted; a light separator configured to separate outputted light outputted from the light source into measurement light irradiated to a subject and reference light; a light generator configured to generate interference light by multiplexing reflected light from the subject and the reference light; a detector configured to detect the interference light; a processor; a memory storing computer-readable instructions therein, the computer-readable instructions, when executed by the processor, causing the optical coherence tomography to perform: forming a tomographic image of the subject based on a result by the detector; a first optical element arranged on a measurement light path so as to be closest to the subject and intersect substantially perpendicularly with an optical axis of the measurement light path, the measurement light path being a light path of the measurement light and the reflected light from the subject; and at least one of second optical elements comprising a pair of surfaces, the pair of surfaces of the at least one of the second optical elements being present in at least one of the light source, the measurement light path, and a reference light path being a light path of the reference light, the pair of surfaces of the at least one of the second optical elements intersecting substantially perpendicularly with an optical axis of the outputted light, the measurement light, or the reference light, wherein the first optical element satisfies at least one of following conditional formulas: 
 −( W−S )< U− 2 Z<X−W; [a2] 
 U− 2 Z<−W; and  [b1] 
 U− 2 Z>X−W+S, [c2] W: a predetermined operation distance being a distance between a subject-side surface of the first optical element and a surface of the subject U: a depth of interest being a depth from the surface of the subject to an area of interest of the subject S: a range of interest being a depth range of the area of interest X: a distance which is greater than W+U+S and minimal among distance(s) between the pair of surfaces of at least one of second optical elements Z: a shallowest position of the area of interest relative to an origin position where a difference between a length of the measurement light path and a length of the reference light path becomes zero.