PURPOSE: To form a nearly arc-like groove by cutting the internal peripheral surface of a cylindrical workpiece while bearing a high load in a simple and miniaturized structure.

CONSTITUTION: A groove forming device for a cylindrical workpiece internal peripheral surface is provided with a tool holding body 14 with a cutter 13 at the head thereof, a pair of link arms 15 and 16 swingably extended from the tool holding body 14, a support shaft 25 to pivotally support the head of the link arm 16 at a base body 11, a first drive crank shaft 17 to pivotally support the head of the link arm 16, and a second drive crank shaft 18 to reciprocatively swing the tool holding body 14 along the internal peripheral surface of a cylindrical workpiece W. The first drive crank shaft 17 turns synchronizing with the second drive crank shaft 18, and the head of the link arm 16 is made to revolve around the turning axis. The cutter 13 generates a nearly arc-like locus when the tool holding body 14 is made swing to the cylindrical workpiece W, and generates a nearly straight line when the tool holding body 14 is made to swing in the returning direction.