Screw shaft adjustment means for a magnetic disk drive unit

A magnetic disk drive unit has a carriage which holds a magnetic head and travels in the radial direction of a magnetic disk, a guide shaft for guiding the carriage, a screw shaft placed in a substantially parallel relationship with the guide shaft and rotated by a motor, and a screw engagement portion of the carriage engaging with the screw shaft. One end of the screw shaft is held in a fixed position, and a bearing member supporting the other end of the screwshaft is adjustable to a certain degree. The stop position accuracy (tract position decision accuracy) of the carriage is thereby improved.