To use a projector anytime anywhere, a lot of projector-camera based approaches have been proposed. Focal correction technique, one of them, reduces the effect of defocus blur which occurs when a screen is located out of projector's depth-of-field. To achieve focal correction, we have to estimate the degree of defocus blur on the displayed images. Some previous methods use any special pattern images. A method which needs no special image has been proposed, however, this method can't estimate at less-textured or already blurred region in the projection image. In this paper, we propose a novel method for estimating PSF (Point Spread Function) without projecting any special pattern images. To prevent such an estimation error, we search and extract well-suited regions to estimate PSF in the projection image and estimate the PSF at extracted region. Experimental results show that our method can prevent PSF estimation error without any special images.