A regulating device including a balance (1) and a plane hairspring (2) for a time piece movement, the plane hairspring (2) including a stiffened portion (8) in its outer turn (7) that is arranged to cause the deformations of the turns to be substantially concentric. The spacing (d) between a terminal portion of the outer turn (7) and the last-but-one turn (9) of the hairspring (2) is large enough for said last-but-one turn (9) to remain free radially during expansions of the hairspring (2) up to amplitudes corresponding substantially to the maximum angle of rotation of the balance (1) in the movement.
