The present invention relates to a seal plug brazing method and an in-core instrument using the same, capable of securely and stably welding an element and an extension tube to a seal plug of an outer sheath tube (OST) of an in-core instrument mounted in a nuclear reactor. The seal plug brazing method includes the steps of: respectively arranging the element and the extension tube in an element hole and an extension tube hole of a header combined with the seal plug; inserting a brazing filler into the extension tube hole and the element hole; and brazing the extension tube and the element by heating the brazing filler at the same time. The extension tube hole and the element hole have gaps with regard to the extension tube and the element. A filler settling part is formed on the opening parts of the element hole and the extension tube hole. The gap between the extension tube hole and the extension tube is 0.03 to 0.07mm. The gap between the element hole and the element is 0.06 to 0.10mm.
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