We set up seventeen types of ALC external walls in three conditions of outdoor temperature (0, -10, -20 ℃ : constant) and three conditions of indoor air humidity (40, 60, 80 % R. H. in 20℃) for thirty days, and measured the quantity of vapor flowed into ALC external walls from indoor and the quantity of internal condensation of ALC external walls. As the outdoor temperature lowered and the indoor air humidity is higher, the quantity of flowing vapor and internal condensation increased. On this point, a simple ALC walls (50mm thickness) is typical. As the outdoor temperature lowered, most of flowing vapor into wall accumulated inside wall.