To improve the operability of a clutch device by keeping it in a engaged or disengaged condition without continuously operating an operation part.

A sliding door 14 has a drum 27 on which a cable 23 to be fixed to a vehicle body is applied, and an electric motor 34 for rotationally driving the drum 27 to give opening/closing operation to the sliding door 14. The clutch device 41 is provided between the drum 27 and the electric motor 34. The clutch device 41 has a driving side disc and a driven side disc provided in an axially engageable and detachable manner. An index plate 44 is provided on an upper side case 18a for driving the driving side disc in the axial direction. The index plate 44 is driven to be rotated at a predetermined angle each time by a switching mechanism 61. It drives the driving side disc in the axial direction each time when rotated at the predetermined angle to alternately switch the clutch device 41 between the engaged and disengaged conditions.