Prevention of dementia is a crucial issue in this aged society. We propose coimagination method for prevention of dementia through supporting interactive communication with images. Coimagination method aims to activate three cognitive functions: episode memory, division of attention, and planning function, which decline at mild cognitive impairment (MCI). Participants of the coimagination program bring images according to the theme and communicate with them. The objective of the program is to make participants to focus on present and future rather than past, which is major difference between coimagination and reminiscence. We measure frequency of comments by others for each participant in order to evaluate interactivity of conversation. They take memory task whether they remember the owner or theme of images after the series of sessions. We held coimagination program successfully at the welfare institution for elderly people in Kashiwa city, Japan. Each session was held one hour per week for five times. The result of the task indicates that the participants showed empathy with each other. The effectiveness of the proposed method was validated through the experiment.