A rotary to linear converter is disclosed as adapted for transporting a magnetic transducer head from track to track on a magnetic disk in a disk drive. Included are an electric stepping motor coupled to a lead screw, and a head carriage engaged with the lead screw for linear travel in response to the rotation thereof. A pin is engaged with a helical groove of the lead screw. The pin is arranged in perpendicular to the lead screw. Consequently, even if distance between the lead screw and a guide rod of the head carriage deviates from a desired value, the pin is not displaced in the extending direction of the lead screw.