1. A method of observing a tomographic image of an examinee's eye to be examined, including: obtaining a first tomographic image which is a tomographic image of an anterior segment of the examinee's eye by an optical coherence tomography optical system for obtaining a tomographic image by guiding a measurement light to the anterior segment of the examinee's eye; obtaining a second tomographic image which is a tomographic image of a fundus of the examinee's eye by optical coherence tomography optical system for obtaining a tomographic image by guiding the measurement light to the fundus of the examinee's eye by a timing different from a timing of obtaining the first tomographic image; constructing a tomographic eyeball image of the examinee's eye based on the obtained first and second tomographic images; and displaying the constructed tomographic eyeball image on a monitor.