A fastening tool for rotating a nut in a specified rotational direction relative to a pin, by engaging with the pin and the nut, the pin having a male thread section and the nut being screwed on to the male thread section of the pin, the pin further having an engaging portion provided at a tip end surface in an axial direction of the male thread section, the fastening tool comprising: a rotational member having a through hole extending along a rotational axis of the nut, the through hole having an opening on one end and an opening on the other end, the opening on the one end of the through hole configured to house the nut such that the one opening on the one end engages with the nut in the specified rotational direction and the engaging portion of the pin exposes in the through hole; a pin engagement member, inserted into the through hole of the rotational member from the opening on the other end, for engaging with the engaging portion of the pin exposed inside the through hole; a tool main body supporting the rotational member coaxially with the nut; and a one-way mechanism, attached to the tool main body, for supporting the pin engagement member coaxially with the nut, the one-way mechanism restricting the pin engagement member from rotating toward the specified rotational direction, and allowing the pin engagement member to rotate toward a direction opposite to the specified rotational direction, wherein the tool main body rotates the nut relative to the pin by driving the rotational member toward the specified rotational direction with respect to the pin engagement member characterized in that the pin engagement member comprises: a cylindrical member (31a, 31b) supported coaxially with the nut by the one-way mechanism; and an L-shaped key wrench having a long shaft section and a short shaft section and a polygonal cross section, said long shaft section being inserted into a polygonal hole in an axial direction of the cylindrical member (31a, 31b), engaging with the cylindrical member (31a, 31b) in the specified rotational direction, and being supported such that it is capable of moving in the direction of the axis of rotation A with respect to the pin engagement member, and is not capable of rotating around the axis of rotation A with respect to the pin engagement member, wherein the L-shaped key wrench engages with the engaging portion of the pin exposed inside the through hole of the rotational member, and the short shaft section extends to the outside. a rotational member having a through hole extending along a rotational axis of the nut, the through hole having an opening on one end and an opening on the other end, the opening on the one end of the through hole configured to house the nut such that the one opening on the one end engages with the nut in the specified rotational direction and the engaging portion of the pin exposes in the through hole; a pin engagement member, inserted into the through hole of the rotational member from the opening on the other end, for engaging with the engaging portion of the pin exposed inside the through hole; a tool main body supporting the rotational member coaxially with the nut; and a one-way mechanism, attached to the tool main body, for supporting the pin engagement member coaxially with the nut, the one-way mechanism restricting the pin engagement member from rotating toward the specified rotational direction, and allowing the pin engagement member to rotate toward a direction opposite to the specified rotational direction, wherein the tool main body rotates the nut relative to the pin by driving the rotational member toward the specified rotational direction with respect to the pin engagement member characterized in that the pin engagement member comprises: a cylindrical member (31a, 31b) supported coaxially with the nut by the one-way mechanism; and an L-shaped key wrench having a long shaft section and a short shaft section and a polygonal cross section, said long shaft section being inserted into a polygonal hole in an axial direction of the cylindrical member (31a, 31b), engaging with the cylindrical member (31a, 31b) in the specified rotational direction, and being supported such that it is capable of moving in the direction of the axis of rotation A with respect to the pin engagement member, and is not capable of rotating around the axis of rotation A with respect to the pin engagement member, wherein the L-shaped key wrench engages with the engaging portion of the pin exposed inside the through hole of the rotational member, and the short shaft section extends to the outside. a cylindrical member (31a, 31b) supported coaxially with the nut by the one-way mechanism; and an L-shaped key wrench having a long shaft section and a short shaft section and a polygonal cross section, said long shaft section being inserted into a polygonal hole in an axial direction of the cylindrical member (31a, 31b), engaging with the cylindrical member (31a, 31b) in the specified rotational direction, and being supported such that it is capable of moving in the direction of the axis of rotation A with respect to the pin engagement member, and is not capable of rotating around the axis of rotation A with respect to the pin engagement member, wherein the L-shaped key wrench engages with the engaging portion of the pin exposed inside the through hole of the rotational member, and the short shaft section extends to the outside.