This is a myth. Ninigi no mikoto was the grandson of the Sun Goddess Amaterasu. He descended from heaven to earth on the mountain Takachiho in Southern Japan (Kyushu). In his hands he held the three sacred treasures: a mirror, a sword and the jewels. His divine mission was to rule over the country.
His great-grandson Kamuyamato iwarehiko, better known as Jimmu-teno, was the first emperor of Japan. According to the Nihongi, Japanese Annals, he ascended the throne on the 1st day of the 1st month of the year Kanoto-tori, which is around 660 B.C . This was to be meant as the end of the ages of the Gods and the beginning of Japan.
In an edict of 1872 the government of Japan designated February 11th to their National Foundation Day referring to the mythical annals. Because of the strong national reference it was abandoned in 1945, but re-established in 1966 under strong national voices and is still celebrated until today.