Abstract: A folding knife is provided with a handle and a blade which is pivotally supported by the handle through a blade shaft. The blade has a tang provided with a cam surface. The cam surface contains a substantially arcuate guide portion and an engage portion formed contiguous to one end of the guide portion. A lock plate is rotatably supported by the handle. The lock plate is rotatable between a lock position where it is engaged with the engage portion and an unlock position where it is disengaged from the engage portion. When the blade is located at an open extended position, a torsion spring rotates the lock plate so as to lock the blade against the handle. Thus, the blade is securely locked at the open extended position.