A fusion splicer comprising: an imaging unit configured to take a lateral transmission image by illuminating a target optical fiber from a lateral direction of the target optical fiber; characterized in that the fusion splicer further comprises: a determination module configured to create a target luminance distribution in a direction perpendicular to an optical axis of the target optical fiber using the lateral transmission image so as to determine a type of the target optical fiber, by comparing the target luminance distribution with preliminarily registered data of a reference luminance distribution of a reference optical fiber; a fusion control module configured to carry out a fusion splicing process for the two optical fibers by using fusion conditions based on types of the two optical fibers determined by the determination module; and a registration module configured to create a message when the target luminance distribution is determined to be outside a tolerance of the reference luminance distribution and to display the message to an operator so that the operator can decide whether or not to register the target luminance distribution of the target optical fiber as a new reference luminance distribution, wherein, when the registration of the target optical fiber is selected in accordance with the message, the registration module is configured to register a new reference luminance distribution by averaging a plurality of luminance distributions which are created using a plurality of new lateral transmission images taken by rotating the target optical fiber, so as to compensate a variation in the luminance distributions caused by eccentricities around the optical axis of the target optical fiber. an imaging unit configured to take a lateral transmission image by illuminating a target optical fiber from a lateral direction of the target optical fiber; characterized in that the fusion splicer further comprises: a determination module configured to create a target luminance distribution in a direction perpendicular to an optical axis of the target optical fiber using the lateral transmission image so as to determine a type of the target optical fiber, by comparing the target luminance distribution with preliminarily registered data of a reference luminance distribution of a reference optical fiber; a fusion control module configured to carry out a fusion splicing process for the two optical fibers by using fusion conditions based on types of the two optical fibers determined by the determination module; and a registration module configured to create a message when the target luminance distribution is determined to be outside a tolerance of the reference luminance distribution and to display the message to an operator so that the operator can decide whether or not to register the target luminance distribution of the target optical fiber as a new reference luminance distribution, wherein, when the registration of the target optical fiber is selected in accordance with the message, the registration module is configured to register a new reference luminance distribution by averaging a plurality of luminance distributions which are created using a plurality of new lateral transmission images taken by rotating the target optical fiber, so as to compensate a variation in the luminance distributions caused by eccentricities around the optical axis of the target optical fiber. a determination module configured to create a target luminance distribution in a direction perpendicular to an optical axis of the target optical fiber using the lateral transmission image so as to determine a type of the target optical fiber, by comparing the target luminance distribution with preliminarily registered data of a reference luminance distribution of a reference optical fiber; a fusion control module configured to carry out a fusion splicing process for the two optical fibers by using fusion conditions based on types of the two optical fibers determined by the determination module; and a registration module configured to create a message when the target luminance distribution is determined to be outside a tolerance of the reference luminance distribution and to display the message to an operator so that the operator can decide whether or not to register the target luminance distribution of the target optical fiber as a new reference luminance distribution, wherein, when the registration of the target optical fiber is selected in accordance with the message, the registration module is configured to register a new reference luminance distribution by averaging a plurality of luminance distributions which are created using a plurality of new lateral transmission images taken by rotating the target optical fiber, so as to compensate a variation in the luminance distributions caused by eccentricities around the optical axis of the target optical fiber.