An instrument comprises a headpiece at a proximal end, a shaft connected to said headpiece, said shaft can be deflected at least in a distal end area thereof. A deflection mechanism is provided for deflecting said shaft. Said deflection mechanism has control wires and a control element for controlling a deflection movement of said deflection mechanism. A lock is provided for locking said deflection mechanism. Said lock has a catch mechanism connected to said control element in such a way that said catch mechanism, without actuation, is automatically forced into a locking position and a movement of said control element first opens said catch mechanism and only then permits a deflection of said shaft. A release of said control element in any position of said deflection mechanism causes an enforced locking by said catch mechanism in said position.