The present invention provides an aptamer binding to midkine and capable of forming a potential secondary structure represented by the formula (I): wherein X1, X2, X5 and X6 are the same or different and each is one or two nucleotides selected from the group consisting of A, G, C, U and T, or a bond, X1 and X6, and X2 and X5 each form a Watson-Crick base pairs, and X3 and X4 are the same or different and each is a nucleotide selected from A, G, C, U and T.