1. A biological-specimen observation apparatus comprising: a stage on which a specimen is to be mounted, the stage being movable to a plurality of positions, wherein the specimen is a small laboratory animal; an objective lens, disposed opposing the stage, that collects observation light from the specimen; a first light source that emits illumination light and irradiates the specimen at an angle with respect to the stage from a side toward the objective lens without passing through the objective lens; a second light source that emits excitation light and irradiates the specimen through the objective lens, along a direction coaxial with the optical axis of the objective lens; an image-forming lens that forms an image from the observation light from the specimen, collected by the objective lens, wherein the observation light comprises a light returning from the specimen as a result of the illumination light and a fluorescence emitted from the specimen as a result of the excitation light; an image acquisition unit that acquires the image of the specimen, formed by the image-forming lens; a position detection unit that obtains a positional information of the stage; an image control unit that moves the stage to a plurality of positions and causes the image acquisition unit acquire the image at each of the plurality of positions; an image storage unit that stores a plurality of the images obtained by the image acquisition unit and the positional information obtained by the position detection unit, the plurality of the images and the positional information being associated to each other; an image processing unit that generates one combined image by combining the plurality of the images stored by the image storage unit on the basis of the positional information stored in association with the plurality of the images; and a diaphragm device configured to change an aperture diameter between the objective lens and the image-forming lens.