In the safety cutter knife provided, a blade slide holding a blade is rotated in the direction nearly perpendicular to the longitudinal center line of the housing of the cutter knife by virtue of a reaction force from a work material when the cutting edge of the blade extended from the housing is pressed into the work material while the blade slide and the actuator slide are located at the extended position. With this rotation of the blade slide, the engagement between the blade slide and the actuator slide is released, such that the blade slide with the blade can return automatically from the extended position to the retracted position by virtue of the energizing force of a spring when the blade is released from the work material.