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 are expected to come to have a predetermined correlative relationship within a predetermined period; and image-storage controlling means for exercising control so that the image data in the predetermined period from the detection is maintained in image-data storage means.