A medical apparatus, comprising: a light source section capable of irradiating an object in a living body with an excitation light and a reference light alternately; an image pickup section that is configured to output a fluorescence image signal and a reference light image signal by picking up fluorescence images of fluorescence that is emanated when a fluorescent substance existing in the object is excited by irradiation of the excitation light, and by picking up reference light images of reflected light that is the reference light reflected in the object, respectively; a storage section that is configured to store in advance information including a target value of a brightness ratio of the fluorescence image and the reference light image that are picked up by the image pickup section, a pixel addition processing section that is configured to perform pixel binning processing of making a signal corresponding to a number of added pixels in the fluorescence image signal obtained by picking up of the fluorescence image of the fluorescence be a signal corresponding to one pixel; a brightness adjusting section that is configured to perform adjustment of brightness so that a brightness ratio of the fluorescence image to which the pixel addition processing is applied and the reference light image obtained by picking up of an image of the reflected light has a constant value, wherein the brightness adjusting section is configured to calculate a color balance adjustment coefficient based on the target value stored in the storage section, and is configured to perform adjustment of brightness by using the calculated color balance adjustment coefficient, wherein, when the number of added pixels in the pixel binning processing is changed, the brightness adjusting section is configured to update the color balance adjustment coefficient based on the changed number of added pixels and the target value, and is configured to perform adjustment of brightness by using the updated color balance adjustment coefficient; and an image processing section that is configured to apply pixel interpolation processing to the fluorescence image signal to which the adjustment of brightness is applied in the brightness adjusting section, and to perform image synthesis so that the fluorescence image signal to which the pixel interpolation processing is applied and the reference light image signal to which the adjustment of brightness is applied in the brightness adjusting section are included in a same observed image. a light source section capable of irradiating an object in a living body with an excitation light and a reference light alternately; an image pickup section that is configured to output a fluorescence image signal and a reference light image signal by picking up fluorescence images of fluorescence that is emanated when a fluorescent substance existing in the object is excited by irradiation of the excitation light, and by picking up reference light images of reflected light that is the reference light reflected in the object, respectively; a storage section that is configured to store in advance information including a target value of a brightness ratio of the fluorescence image and the reference light image that are picked up by the image pickup section, a pixel addition processing section that is configured to perform pixel binning processing of making a signal corresponding to a number of added pixels in the fluorescence image signal obtained by picking up of the fluorescence image of the fluorescence be a signal corresponding to one pixel; a brightness adjusting section that is configured to perform adjustment of brightness so that a brightness ratio of the fluorescence image to which the pixel addition processing is applied and the reference light image obtained by picking up of an image of the reflected light has a constant value, wherein the brightness adjusting section is configured to calculate a color balance adjustment coefficient based on the target value stored in the storage section, and is configured to perform adjustment of brightness by using the calculated color balance adjustment coefficient, wherein, when the number of added pixels in the pixel binning processing is changed, the brightness adjusting section is configured to update the color balance adjustment coefficient based on the changed number of added pixels and the target value, and is configured to perform adjustment of brightness by using the updated color balance adjustment coefficient; and an image processing section that is configured to apply pixel interpolation processing to the fluorescence image signal to which the adjustment of brightness is applied in the brightness adjusting section, and to perform image synthesis so that the fluorescence image signal to which the pixel interpolation processing is applied and the reference light image signal to which the adjustment of brightness is applied in the brightness adjusting section are included in a same observed image.