A lubrication device including a bypass valve with an inlet connected to a feed circuit, a first outlet to be connected to elements to be lubricated, and a second outlet connected to a bypass circuit. The valve includes a valve member slidable between first and second positions, and separating a cavity into first and second chambers. The first chamber is connected to the inlet. In the first position, a fluid flow passage from the first chamber to the second chamber is open, and from the first chamber to the first outlet is closed. In the second position, a fluid flow passage from the first chamber to the first outlet is open, and from the first chamber to the second outlet is closed. At least in the second position, the valve member separates the chambers in substantially leaktight manner, while the second chamber remains in fluid flow communication with the second outlet.