The purpose of this research is reducing the cracks produced in the plastering mortar that has contraction restrained by ground concrete about the outer wall and floor component of reinforced concrete buildings. It tried to evaluate the relation between the construction time of plastering mortar and the small crack produced on the mortar surface by this paper. The examination object which finished in the middle of the month every month of one year, and constructed mortar was exposed to the outdoors and indoor using the small concrete with plastering mortar specimens, and the generating situation of a crack was observed. Consequently, it was experimentally shown clearly that small cracks become easy to produce the time when temperature falls in one month after the mortar plastering.
