The carp flag refers to the carp yue longmen, which originated in China. Legend has it that every March and April, carp will gather in Longmen of the Yellow River. If you can go upstream and cross the waterfalls in Beishan, you can become a dragon. Therefore, the Japanese think carp is a symbol of good luck. In order to pray for God to bless their children, they put up the carp flag to attract God's attention.

The carp flag is a hollow carp made of cloth or silk. There are three colors: black for father, red for mother and blue for boy. The number of blue flags represents the number of boys.

The meaning of the carp flag comes from the story of carp yue longmen Waterfall in China's official history "The Book of the Later Han Dynasty", and carp yue longmen became a symbol of success.

The carp flag in the Edo period was only herring, but from the Meiji period, herring and red carp hung in pairs, and from the Showa period, a small green carp was added to show the family.