PURPOSE: To surely connect a main tube with a branch tube, to prevent a trouble of a gas or a cold/hot water from leaking from a connecting part and also, to provide a copper tube burring method and the device that the branch tube of a diameter as same as the main tube can be connected.

CONSTITUTION: The device is composed of a hole piercing tool 8 piercing a prepared hole 1b on an outer circumferential surface 1a of a main tube 1, a prepared hole enlarging tool 9 that an engaging hole 9c is formed facing from a conical shape top end part 9a to a rear end part 9b side and a connecting shaft 10 to be engaged to this engaging hole 9c is provided, and an opening enlarging tool 12 that the top end part 13 is a conical shape and the base end part 14 is made to a circular column shape, a large diameter part is provided at this base end part 14 side and a cutting blade whose blade top is extended to the outer diameter direction is formed on the front surface side of this large diameter part.