The object of the present invention is to provide a follistatin variant capable of specifically inhibiting GDF-8 activity without inhibiting activin activity. The present invention provides a follistatin variant polypeptide, which (a) comprises follistatin domain I; (b) does not comprise an amino acid sequence that is represented by formula (I): Cys-(X1)a-Cys-(X2)b-Cys-(X3)c-Cys-(X4)d-Cys-(X5)e-Cys-(X6)f-Cys-(X7)g-Cys-(X8)h-Cys-(X9)i-Cys (SEQ ID NO: 30) wherein X1, X2, X3, X4, X5, X6, X7, X8, and X9 each independently represents the same or a different naturally occurring amino acid residue other than cysteine, “a” represents an integer between 2 and 6, “b” represents an integer between 3 and 7, “c” represents an integer between 7 and 11, “d” represents an integer between 0 and 4, “e” represents an integer between 1 and 6, “f” and “g” represent integers between 8 and 12, “h” represents an integer between 4 and 8, and “i” represents an integer between 11 and 15) and has 50% or more homology with the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 29; and (c) selectively inhibits GDF-8 activity as compared with its inhibition of activin activity.