1. A fluorescence observation apparatus, comprising: a light source configured to irradiate a subject with reference light and excitation light; and one or more processors comprising hardware, wherein the one or more processers are configured to implement: a fluorescence-image generating portion configured to generate a fluorescence image based on fluorescence produced in the subject due to irradiation with the excitation light; a reference-image generating portion configured to generate a color reference image based on return light returning from the subject due to irradiation with the reference light; an extraction portion configured to extract, from the fluorescence image generated by the fluorescence-image generating portion, a fluorescence region having a gradation value equal to or higher than a prescribed threshold; a motion-effect generating portion configured to generate a frame image that is assigned a time-varying gradation value so as to bring about an effect that is visually time-varying with a variation quantity depending on a gradation value of the fluorescence region, in a region corresponding to the fluorescence region extracted by the extraction portion; and a synthesis portion configured to add the frame image generated by the motion-effect generating portion to at least one of a plurality of color-component images constituting the reference image, to synthesize a synthetic image from the plurality of color-component images comprising the color-component image that the frame image has been added to.