Queen (of imperial family) (女王 (皇族))
A queen (of imperial family) is a female who is in the direct line and in the third or more degree of kinship from the emperor (not in a collateral line, and counted from the emperor who was a direct ancestor) according to the Imperial House Law. Or it indicates her rank or her title.
Today, a total of five princesses, Princess Akiko and Princess Yoko, who are daughters of Imperial Prince Tomohito, Princess Tsuguko, Princess Noriko, and Princess Ayako, who are daughters of Takamadonomiya Imperial Prince Norihito, have that position.
Summary
In the ritsuryo system, those who were in a direct line and in the up to six degree of kinship were the Imperial family, but there is no such restriction after the Meiji period. When a queen becomes an adult, she receives the Order of the Precious Crown, Peony (or if she became an adult before November 2, 2003, she received the Second Order of the Precious Crown).
These five queens are in the three degree of kinship, i.e., grandchildren of Emperor Taisho who was their direct ancestor.