TOKYO — Japan’s Parliament opened the door to modernizing the Chrysanthemum Throne on Friday, unanimously passing a special law allowing the ailing Emperor Akihito to retire, and proposing that the government consider letting the royal bloodline pass through the women of the imperial family.

At a time when Prime Minister Shinzo Abe is promoting a political message of women’s empowerment, the rules of succession governing the world’s oldest monarchy do not permit women to be put in line for the throne.

But with public pressure mounting to allow women to reign, the shrinking size of the imperial household — and the dwindling number of male heirs — has pushed the question of the role of royal women to the forefront of the abdication debate.

Ever since Emperor Akihito, 83, announced his desire to abdicate on television last August, the Parliament has deliberated over whether to grant his request. With the passage of the special bill by the upper house on Friday, the stage is now set for the emperor to step down and pass the throne to his eldest son, Crown Prince Naruhito. The lower house passed the legislation last week.