A timepiece, in particular a wristwatch, includes a constant-force device for acting on an oscillating system, the constant-force device including a switching unit and a tensioning module having a primary spring-energy store and a secondary spring-energy store. A seconds shaft bearing a seconds wheel and a cam plate is driven to rotate constantly by the secondary spring-energy store. The seconds wheel drives the oscillating system, and the cam plate acts on the switching unit for the periodic tensioning of the secondary spring-energy store by the primary spring-energy store.