To improve assembly work efficiency and to reduce a cost by a method wherein this device forms simple constitution.

In a groove machining device 1 to machine a groove 2c in the hole 2b of a work 2, this machining device comprises a holding member 3 for a work 2; a crank member 4', a cutting part 5c arranged at one end part 5b thereof; a cutting member 5 to rotatably couple the other end part 5a to the crank pin 4a of the crank member 4; and an arm member 6 to oscillatorily support one end part 6a around a shaft part 11 and support the intermediate part 5d of the cutting member 5 such that a cutting member 5 is oscillatory centering around an intermediate part 5d.