1. An image storage device comprising: facial-expression-correlation estimating means for estimating a facial expression correlation representing a degree of correlation between a facial expression in a face image and a predetermined reference facial expression, the face image being included in time-varying image data; facial-expression estimating means for detecting, through estimation based on transition of the facial expression correlation, a situation where the facial expression in the face image and the predetermined reference facial expression have a predetermined correlative relationship including a (i) gradient of change in the facial expression correlation exceeds a predetermined value or (ii) the facial expression correlation exceeds a predetermined degree, within a predetermined period; and image-storage controlling means for exercising control so that a rapid fire image shooting and storing is performed during a given period, when the predetermined correlative relationship is detected by the facial-expression estimating means, wherein the facial-expression estimating means continuously detects, while the image-storing controlling means exercises control to perform rapid fire image shooting and storing, for the situation where the facial expression in the face image and the predetermine reference facial expression have a predetermined correlative relationship.