18+ Kuro-senpai to Kuroyashiki no Yami ni Mayowanai

Reiichi heads off for a part-time job interview, hoping to get money for a new bike, and his self-styled servant Kuro-senpai tags along. The "interview" is in a western-style mansion, but no one seems to be around. The two of them end up locked in the mansion and must find their way out!

(Source: MangaHelpers)