To provide a groove-machining device capable of correctly machining a groove of a predetermined shape in a side face of a tubular member by a simple mechanism by maintaining flatness (roundness) of an outer circumference and suppressing occurrence of slump in an edge part of the groove without using cutting to generate chips or without inserting a core in the diameter of the tubular member.

The groove-machining device has an idler roller 1 with a projecting bar tooth 1a having a contour shape corresponding to the predetermined groove shape formed on a circumferential surface 1b in slidable contact with a side surface of a tubular member P, the idler roller 1 is held by a roller holder 1d moving from the side of the orthogonal direction toward the axis of the tubular member P, and the idler roller 1 is provided as a groove forming means by the projecting bar tooth 1a.

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