A pipe-fitting device according to the present invention includes a fitting body, a fixing nut, and a collet. A first end of the fitting body is rotatably connected to an outer circumferential surface of a first pipe, and a second end of the fitting body surrounds an outer surface of a second pipe. A first end of the fixing nut is rotatably connected to an outer circumferential surface of the fitting body, and a second end of the fixing nut surrounds the outer surface of the second pipe. The collet is provided inside the fixing nut and presses the second pipe when the fixing nut is rotated and tightened. The collet includes a seating portion provided inside the fixing nut, a bending portion in which a bending groove is formed, a pressing portion which presses and seals the second pipe, and a protecting portion which protects the pressing portion.