To provide a unit building capable of easily forming a boundary wall in a building field.

A gap among a first cot side wall surface material 51A and a third floor side wall surface material 51B forming a first vertical intermediate boundary wall 51, a beam 41A stored in the interior, a short floor beam 14B and a short ceiling beam 13B is filled with a rock wool 63, and a gap between the beam 41A stored in the interior of the first vertical intermediate boundary wall 51 and the short floor beam and a gap up to the short ceiling beam 13B are filled with a rock wool 63. Even if flame of a fire enters the first vertical intermediate boundary wall 51, invasion to the third floor building unit 30A can be prevented by the rock wool 63, so that it is not necessary to give the same fire protection as the first vertical intermediate boundary wall 51 to the interior facing material 31A. The above structure can be simply formed so as to easily form a boundary wall 50 in a building field.