To provide a mounting structure of a post core capable of adjusting the mounting state of the posts of a handrail installed in a veranda or a balcony when the posts are installed.

An anchor plate 1 is attached to the bottom end of the post core 3 by inserting a bolt 4 through an elongated hole 1a and tightening the bolt. Even if the position of an insertion hole which is formed in the veranda and through which the anchor 2 of the anchor plate 1 is inserted is slightly incorrect, the post core 3 can be arranged at an appropriate position by relatively displacing the mounting positions of the anchor plate 1 and the post core 3 using the elongated hole 1a. The perpendicularity of the post core 3 is ensured by adjusting the interface between the anchor 2 and the insertion hole through which the anchor 2 is inserted. A predetermined perpendicularity is ensured at a predetermined position, a fixing material such as mortar is placed between the post core 3 and the veranda to firmly affix the post core 3. Finally, the post is fitted into the post core 3.