Provided is an opening/closing member drive device wherein an opening/closing member can be moved in a drive direction by means of mild force when the opening/closing member is being driven. Specifically, an automatic closing device 1 that has a gear mechanism 21 that transmits the drive force of a motor 20 to an output shaft 11 that is connected to an opening/closing member 2. The output shaft 11 comprises a contacted part 108a that is separated in the radial direction from a rotational center line L0. An output gear 27 of the gear mechanism 21 comprises a contact part 105a that, when the output gear 27 rotates in a first rotation direction S1, contacts the contacted part 108a from behind in the first rotation direction S1 and makes the output shaft 11 rotate therewith. The output shaft 11 does not engage the output gear 27 and rotate, but rather co-rotates with the output gear 27 as a result of the contact part 105a contacting the contacted part 108a. When the opening/closing member 2 is operated by manpower and moved in a closing direction, only the output shaft 11, which is connected to the opening/closing member 2, rotates in the first rotation direction S1.