A method of determining a heating amount adequate for fusion splicing is provided. In the method, the melting state of the end portions of optical fibers can be monitored on a real time basis so that fewer tests need to be performed. A method of fusion splicing and a fusion splicer are also provided. In the method of determining the heating amount, end portions of optical fibers that are placed opposite one another with a predetermined gap therebetween are heat-melted; an image of portions to be heat-melted is observed with an image-capturing device; and a luminance, a light emitting width, or a change in the luminance or the light emitting width is measured. In the method of fusion splicing, optical fibers are heat-melted with the heating amount that is determined using test fibers in advance, or determined using the optical fibers to be fusion spliced.