Ninomiya was born in Katsushika, Tokyo as the youngest child of his family; his sister is two years older than he is. His father and mother were both working as chefs at Hattori Nutrition College when they met, his father being a Japanese cuisine teacher, and his mother, his assistant. When Ninomiya was born, his grandfather immediately came home and named him the heir to the family's windshield wiper factory, since he was his grandfather's only grandson.
Ninomiya graduated from high school in March 2002 at the age of 18.
