"Okujou de kossori kuwaeru maisen" is translated as "I stealthily hold a Meissen in my mouth on the rooftop" and a note explains that a Meissen is a type of procelain work, often a doll. While the note is absolutely true, and meissen porcelain is a famous type of German porcelain, the マイセン mentioned in the song is short for Mairudo Sebun, or Mild Seven, a popular type of Japanese cigarettes.
He's sneaking up to the rooftop to have a smoke, not to hold a procelain doll in his mouth.