僕だけのロマンチスト•Boku dake no Romantist

Reincarnated as the Villain: An Archdemon Fell in Love With Me

Shinobu Aizawa is already in his forties, but he works part-time as a security guard while pursuing his dream of becoming an archaeologist. One day, he is sent to work at a royal party venue and steps in to stop a fight, only the guy responsible for making the fuss turns out to be Aldo, the very prince who organized the party. Before Shinobu has a chance to understand what's going on, he's being dragged into the center of the room and introduced as the prince's fiance! This is a romantic love story between a bold and brazen young prince and a middle-aged dreamer by the popular manga artist Nerima zim! (Source: Renta!)