A locking device for flaps or doors. A door or flap lock serves as a protection against collisions of objects or persons with moving or rotating machine parts, with the possibility of unlocking the door or flap lock by manual operation. A movable bolt is manually operable via a lever, handle, or knob. The bolt includes several engagement elements for interlocking with a housing- or frame-anchored receiving element. The interlocking engagement and receiving elements make it necessary at least on opening the flap or door to repeatedly operate the lever, handle, or knob in at least two opposite directions of movement, pivot, or turning.