A locating pin is mounted on a hollow post member in such a manner that a slit of the locating pin is communicated with an interior of the post member. A clamp arm is axially movably received in the slit and has a hook-shaped head exposed to an upper portion of the slit and a base portion exposed to the interior of the post member. The clamp arm is axially movable between a clamp position wherein the hook-shaped head is projected outward from the upper portion of the slit and an unclamp position wherein the hook-shaped head is retracted in the upper portion of the slit. A guard structure is installed in the slit of the locating pin to isolate the interior of the post member from the interior of the slit while permitting the axial movement of the clamp arm in the slit.