PURPOSE: To prevent an unexpected damage by an external force, a risk by the mischief of an infant, and the like, in a clasp knife using condition, and to obtain an excellent service-ability, by fixing a key plate to a grip to hold hardly removed from a deep groove, and making to release the fitting of the key plate and a notch by pressing a push-button by a finger.

CONSTITUTION: In this safety device, the fitting of a key plate 31 and a notch 23 can be released by pressing a push-button by a finger. That is, a pair of notches 22 and 23; a push-button 3; and a blank cap 4; are provided, and the push-button 3 which consists of the key plate 31 and a projection rod 32 is supported elastically in the deep groove 11 of a grip 1 at the developed position of a blade. In this case, since the key plate 31 is engaged to one side notch 22 of the blade, the blade is fixed to the grip 1 solidly. As a result, even though an external force is applied to the blade, the fingers grasping the grip 1 never receive a damage resulting from the external force at that time. When it is necessary to fold the knife, the elestic support is released by pressing the bush- button 3, and the key plate 31 is removed from the notch 22, so as to make the blade foldable.