1. A fundus oculi observation device comprising: a first image forming part configured to form a 2-dimensional image of a surface of a fundus oculi of an eye; a second image forming part configured to form a tomographic image having a cross-sectional position in a measurement region of the fundus oculi corresponding to a partial region of the 2-dimensional image; a display; a storage configured to store imaging condition information including characteristic information showing a characteristic of the eye; and a controller configured to match display sizes of the formed 2-dimensional image and a measurement range image showing a range of the measurement region with each other by changing the display size of at least the formed 2-dimensional image to match the display size of the measurement range image, based on the stored imaging condition information, and cause the display to display the 2-dimensional image and the measurement range image whose display sizes are matched.