To provide a wall-to-wall mold installed between walls and not deteriorating construction efficiency when a new wall in proximity to an existing wall or new walls in proximity to each other are constructed and a method of constructing a proximity wall using the mold.

When a new wall 4 in proximity to an existing wall 3 is constructed, a wall-to-wall mold A formed of a biodegradable synthetic resin foam body is disposed between the existing wall and the new wall, a general frame B is disposed on one side of the new wall, and concrete is deposited in the mold. After the concrete is cured, an alkaline solution and a bacteria-containing water are sprayed on the wall-to-wall mold left undisturbed. The solution and the water flow into the entire mold through a groove 2, and promote the decomposition of the wall-to-wall mold.