I was a programmer at a black company, but I was hit by a truck and isekai'd. At that time, a sage's soul possessed my head. The sage's soul has unfinished business: "Defeating the Demon King." With the sage's soul's super-Spartan guidance, I trained for 30 years and became known as a hero. Then, I challenged and defeated the Demon King! I was sure the sage's soul would finally rest in peace. I was certain of a slow life in another world, complete with a harem. However, the moment I opened the Demon King's treasure chest, I was sent back to modern-day Japan. Back to one minute before I was hit by the truck. I couldn't bring back any magic or items, but the skills I gained from defeating the Demon King remained. This is the story of a demon king slayer's unrivaled success in the business world after returning from another world. And he hasn't given up on his harem. His dream is big: to start the world's greatest company!