Abstract: This application provides: an antibody or an antigen-binding fragment thereof which can be used not only as a therapeutic drug but as a tool of immunological and immunohistological analysis such as immunostaining or Western blot, specifically recognizes an extracellular region polypeptide of human, mouse and rat ALK2, and inhibits BMP signal transduction, and an antibody or an antigen-binding fragment thereof which has a property of specifically binding to an extracellular region of ALK2 to form an ALK2 homodimer and a property of inhibiting the formation of an ALK2-type II receptor heterodimer, and inhibits the activation of ALK2 kinase, or inhibits BMP signal transduction; and a method for inhibiting the activation of ALK2 kinase or a method for inhibiting BMP signal transduction using the antibody or the antigen-binding fragment thereof.