Recently, the bake-out is one of the indoor air pollution countermeasure, and it has been suggested that newly constructed residence building undergo a bake-out. The purposes of this study were to evaluate the method for the reduction effect of bake-out on formaldehyde, and suggest the guideline of bake-out for the practice use. The experiments were carried out in 3 households, one of these evaluated naturally reduction rate on formaldehyde without bake-out, the rest measured reduction rate by doing bake-out. The temperature and relative humidity were measured during bake-out, and took a formaldehyde sample. The results show as follows; It showed that grew in temperature the section of rising at the rate of $2^{circ}C$ per an hour, and fallen in temperature the section of descending at rate of $0.04^{circ}C$ per an hour. And, in the reduction effect which was estimated based on the concentration transition before and after bake-out, it was confirmed that had a crucial role in the emission pattern of formaldehyde in step of after bake-out. The newly house that produced the high level of formaldehyde concentration, it found out that more available to evaluate for using the accumulated emission in the section of rising temperature during bake-out.