Acting as the monarch

Iｍperial Family system is a state system with the emperor acting as the monarch.
(皇室制度は天皇を君主又は象徴とする国家体制）
 Imperial n)皇室の  ｲﾝペﾘｱﾙ
 state adj)国家の　n)州、国　ｽテｲﾄｩ
 act v)行動する、振舞う　アｸﾄｩ
 monarch n)君主、支配者　モﾅｰｸ

a Japanese monarch 日本の君主

Japan has both a Prime Minister and an Emperor.
 Emperor Hirohito　昭和天皇
 Emperor Akihito　平成天皇