An adjustment unit has a driver for turning an adjustment screw, a sleeve having a hexagonal hole connected to a rotating shaft, an insertion member connected to the upper end of the driver and of which a part is inserted in the sleeve, a pipe concentrically provided around the driver and having at its forward end a socket for rotating an adjustment nut, and a gear body provided concentric with the pipe and rotating the pipe. The sleeve has a short tiltable support section in contact with the hexagonal hole. A gap is provided between the hexagonal hole of the gear body and a hexagonal column section of the pipe. The driver and the pipe tilt about the tilting support section as the fulcrum.