Abstract: A knife apparatus having a knife handle of a substantially C-shaped cross-section defining a groove extending in a longitudinal direction of the handle, a slider slidably received by the groove of the handle and an elongated cutting blade secured to the slider so as to be extended and retracted as the slider is slided along the length of the groove. The knife handle is provided with a series of notches formed in one of the side walls thereof. The slider has a depressable resilient tab extending therefrom in the opposite direction to the cutting blade. The depressable resilient tab is provided with a pawl adapted to be received by selected one of the notches, when the depressable resilient tab is not depressed, so as to fix the slider against the sliding movement. As the depressable resilient tab is depressed, the pawl is disengaged from the notch to allow the slider to be freely slided. The body of the slider and the depressable resilient tab are unitarily formed of a plastic.