In an OCT imaging, a focus position in a tomographic image is determined by a first distance, a second adjustment amount, a third adjustment amount and a refractive index of a medium. The first distance is a distance between a first surface and a second surface of a wall part of a container. The second adjustment amount is a focus position adjustment amount of an objective optical system at which an intensity of reflected light from the second surface is maximized when a reference mirror is positioned at a position where an object optical path length to the first surface and a reference optical path length are equal in a condition that the objective optical system is focused on the first surface. The third adjustment amount is a focus position adjustment amount in the imaging.
