To improve airtightness and heat insulating performance in a guide block of a paper sliding door.

In a guide block 10 of a paper sliding door arranged in a guide groove 1a formed on the upper end of a stile 1, a vertical packing plate 20 composed of a flexible raw material is arranged in a recessed groove part formed in the guide block 10 so as to approach or abut to a guide rail. Horizontal packing pieces 40, 40 are arranged in both shoulder parts of the recessed groove part along the inside of the recessed groove part formed in the guide block 10 so that the loose edge abuts to the side surface of the guide rail. Engaging recessed parts are formed in both shoulder parts of the recessed groove part, and engaging root parts formed in the respective horizontal packing pieces 40 are detachably engaged with the engaging recessed parts.