Digital signage has been becoming more popular due to the recent development of underlying hardware technology and improvement in installing environments. In digital signage, it is important to make the content more attractive to the viewers by evaluating its current attractiveness on the fly, in order to deliver the message from the sender more effectively. Most previous works for this evaluation do not take the viewers' feeling towards the content into account, and the content is improved manually if needed in an off-line manner. In this paper, we present a novel method which does not rely on such manual evaluation and automatically makes the content more adapted to the viewers. To this end, we take advantage of the viewers' involuntary behaviors in front of the digital signage for online updates through the usage of a genetic algorithm.