Lucifer, the strongest Demon king has ruled the demon race with his overwhelming power. Making a Harem with his dearest wife, he spent several hundred years eating, making love and sleeping. Tired of sleeping, the Demon King began to seek for a powerful enemy. Why doesn’t the hero come to subdue him? Even though he’s been waiting for so long. Lucifer then turned himself into a human, and decided to go and see the military academy, which is a training institution for the heroes. The best way to know what’s inside is to enroll in the school. He quickly passed the entrance exam and enrolled safely…!? This is a story of the most powerful demon king that becomes the strongest hero by mistake.