A drive device includes: connection gears connected to driving objects; a movable gear movable between the connection gears; a drive source for rotating the movable gear; and at least one estimator each provided for a corresponding one of at least one of the connection gears and configured to: estimate a reaction-force estimate value, as an estimate value of a reaction force acting in a power transmission system from the drive source to the driving object, based on control input, control output, and a model of the power transmission system; and output the reaction-force estimate value. The model is configured such that the reaction-force estimate value falls within a set range in a state in which the movable gear is meshed with the connection gear. A controller detects mesh and/or separation between the movable gear and the at least one connection gear based on the reaction-force estimate value.