It's a flag made of paper, fabric and nonwoven fabric with a carp design on it, whereby a streamer swings in the wind in the shape of a carp, and it's decorated in the yard to wish for a male child's success on a rainy day during the rainy season until May 5 (old lunar calendar) of the Boys' Festival.
端午の節句である旧暦の5月5日(旧暦)までの梅雨の時期の雨の日に、男児の出世を願って家庭の庭先で飾られた紙・布・不織布などにコイの絵柄を描き、風をはらませてなびかせる吹流しをコイの形に模して作ったのぼり。