In this paper, we defined the period until the accumulated income reaches a maximum as the “commercial life” of a book or digital book after it has been issued, and we analyzed this. First, we defined the lapsed time after the market introduction of a book, and the relation of the number of sales volumes as a “sales function”, and formulized the profits of the publisher in consideration of sales proceeds, production costs, the amount of royalties, and inventory costs as a “profits function” using the sales function. Moreover, we analyzed how individual books would end the commercial life using a profits function, and we showed a conflict of interests occurred between a publishing company and a copyright holder after a commercial life using profits function.
Next, from the viewpoint of the social utility of a copyright holder and an end user, when a copyright holder abandoned copyright at the time of the end of the publication period of each book or digital book, although there was no change in a copyright holder's utility, it was shown that a user's utility becomes large.

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