To shorten a time lag when connecting a gear mechanism in a running mode immediately after starting to a constant speed mechanism, to stabilize slow start, to realize engagement of the constant speed mechanism hard to be removed, and to make a clear switching between slow running and quick running for a long time use.
In this two-speed power spring type driving unit, a counter-side gear 10 for giving rotation to constant speed running switching gears 16 and 17 of the gear mechanism 2 is arranged between a driving shaft 4 and an output shaft 5, and the constant speed running switching gears 16 and 17 rotate around the counter-side gear 10 for giving rotation. When rotating for running, the constant speed running switching gears 16 and 17 are supported freely to move in a direction for engaging with the constant speed mechanism 3 by the rotating force to be generated by the rotation of the counter-side gear 10 and gravity, and the constant speed running switching gears 16 and 17 are moved inside a long hole 23 by a switching member 6 straightly moving simultaneously with a start of the running rotation, resisting the gravity and the rotating force, to be separated from the constant speed mechanism 3.