Abstract: A utility knife having a holder body for supporting a blade slidably between a protruded position and a returned position, the utility knife comprising a spring means to be operated against a slider supporting the blade, a slider for manually moving said blade to a protruded position, a lock means for locking the slide movement of said slider at said protruded position resisting against the spring force of the spring means, and a means for releasing the lock of said lock means, wherein upon use, the blade temporarily fixed to the slider will be protruded from the holder by manually sliding said slider, and the blade could be locked at the end position of the slide. When storing the blade, the lock could be released by pressing on the lock release button, which allows the slider to be pulled back by the spring, and along with this movement, the blade could be stored automatically inside the holder body.