To use a projector anytime anywhere, a lot of projector-camera based approaches have been proposed. In this paper, we focus on a focal correction technique, one of projector-camera based approaches, reduces an effect of defocus blur which occurs when a screen is located out of projector's depth-of-field. We propose a novel method for estimating projector defocus blur on a planar screen without any special measuring images. To estimate a degree of defocus blur accurate, we extract sub-region which is well-suited for defocus blur estimation in a projection image and estimate the degree of defocus blur at each extracted region. To remove the degradation caused by the defocus blur, we pre-correct the projection image depends on the degree of defocus blur before projection. Experimental results show that our method can estimate the degree of defocus blur without projecting any special images and pre-corrected image can reduce the effect of the defocus blur.