Two subject images shot by an imaging section before real exposure are captured, and the difference between data is obtained for each corresponding pixels of the subject images by an interframe difference calculation section. The difference is compared with a threshold value by a threshold value comparison section, the pixels of levels not less than the threshold value level are extracted as a non-correlated portion, and the ratio of the area of the non-correlated portion to all the detected pixels is obtained by a non-correlated portion area calculation section. When the ratio of the area of the non-correlated portion is high, it is determined that the subject movement is large, so that a speed priority program line is selected by an exposure calculation block and the exposure time is set to be shorter than the normal exposure time, thereby preventing subject image blur.