A counter balance device for a sash window includes a spiral balance having a hand-wound spiral rod connected to wind a torsion spring to generate an initial spring force that counter balances the window's weight for keeping the sash up when raised. The lower end of the spiral rod is connected to a rotation shaft rotatably disposed on a slide block. When a large repulsion force produced by the torsion spring is applied to the spiral rod, the rotation shaft is urged in a direction reverse to the rotation direction of the spiral rod by a buffer spring having a spring force which is larger than the initial spring force so that the rotation of the rotation shaft is tolerated.