To obtain a joining composition which is hardened by a prescribed method for bonding and finishing at a joint between dry plaster wall panels and which is free from a thinning phenomenon when filled into a joint and dried, by blending a specified compound therewith.

This composition is obtained by blending a synthetic resin powder putty, which is to be blended with water at an appropriate ratio to produce a moderate stickiness and cured at room temperature, with sodium hydrogencarbonate preferably at a ratio of 100:(3 to 5). This composition, mixed with water preferably at a ratio of 100:(45 to 55), is kneaded to give a filling material 13 to be filled into a joint T between dry plaster wall panels 5, 6. This composition produces carbon dioxide bubbles inside due to spontaneous drying after being filled and expands in volume so that it is free from a thinning phenomenon. The part outside the joint is removed with a sandpaper to accomplish working.