A press machine includes a first rotary shaft which rotates in synchronism with a press crankshaft, a second rotary shaft which is coaxial with the first shaft and which has a drive cam for actuating a knock-out pin, and a clutch for connecting and disconnecting the first and second shafts. Knock-out timing is adjusted by releasing the clutch to separate the shafts and rotating one of the shafts through a predetermined angle relative to the other to produce a predetermined rotational phase difference between them. In the preferred embodiments, the second shaft is axially aligned with the first shaft, and is rotatable but axially immovable with respect to the press frame. A driving gear and a driven gear are provided between adjacent portions of the first and second shafts. The second shaft has a bore in which a lightweight, axially movable shaft is mounted. This movable shaft carries at one end a connector gear which is engageable with the driving and driven gears between the first and second shafts and which, together with the driving and driven gears form the clutch. A connector is provided for engaging the clutch and a separator is provided for disengaging clutch. The first and second shafts are separated by displacing the movable shaft and the connecting gear therewith with the use of the separator to disconnect the driving gear from the driven gear.