Abstract: A knife includes a casing and a blade which can have two positions, an extended position in which the blade projects outside the casing and a retracted position in which the blade is disposed within the casing, a blade carrier to which is connected the blade, a housing on which is slidably mounted the blade carrier and that is removably mounted within the casing, and a finger piece-pusher mounted slidably on the casing to move forwardly the blade carrier to place the blade in a cutting position. The blade carrier and finger piece-pusher are returned to the retracted position by return springs. Components are provided to secure the finger piece-pusher and the blade carrier together to place the blade in the extended position. The components are uncoupled by a conjoint action of rotation and extraction of the blade carrier relative to the finger piece-pusher, resulting from the cutting action.