To realize constitution low in cost, making low operating sound and allowing smooth winding even if a sheet to be wound is thin or light while saving a person from a big ache even in case a part of the person is pinched.

This sheet opening/closing driving device opens/closes the sheet 101 by rewinding and winding the sheet 101 by the rotation of a winding drum 122 driven by a motor 140 and a speed reducer 160. The speed reducer 160 comprises a simple planetary roller mechanism 161 provided with a sun roller 162, a planetary roller 163 coming in rotational contact with the sun roller 162, and a ring roller 164 with which the planetary roller 163 comes in rotational contact.