A fastening structure configured for fastening a flexible externally toothed gear of a strain wave gearing to a shaft member, the fastening structure comprising: an annular or circular boss-side fastening surface formed on an end surface of a rigid boss in the flexible externally toothed gear; a shaft-side fastening surface formed on an end surface of the shaft member to be fastened to fix to the boss coaxially; and a fastener for fixedly fastening the boss and the shaft member along a direction of a center axis line in a state in which the boss-side fastening surface is in coaxial contact with the shaft-side fastening surface, characterized in that one fastening surface (12, 13) which is either one of the boss-side fastening surface and the shaft-side fastening surface is a convex fastening surface defined by first and second inclined surfaces (12a, 12b), the first and second inclined surfaces (12a, 12b) being intersected at a prescribed angle (θ12a, θ12b) to form a ridge line along a diameter of the one fastening surface (12, 13), and the first and second inclined surfaces (12a, 12b) being symmetrical with each other with respect to a plane including the ridge line and the center axis line; the other fastening surface (13, 12) of the boss-side fastening surface and the shaft-side fastening surface is a concave fastening surface defined by third and fourth inclined surfaces (13a, 13b), the third and fourth inclined surfaces (13a, 13b) being intersected at a prescribed angle to form a trough line along a diameter of the other fastening surface (13, 12), and the third and fourth inclined surfaces (13a, 13b) being symmetrical with each other with respect to a flat plane including the trough line and the center axis line; and an inclination angle (θ13a, θ13b) of each of the first to fourth inclined surfaces (13a, 13b) with respect to an axis-orthogonal plane has a value in a range between 2° and 16°, where the axis-orthogonal plane is a plane orthogonal to the center axis line; and the first and second inclined surfaces (12a, 12b) defining the convex fastening surface have an inclination angle (θ12a, θ12b) that is smaller by an amount ranging from 0.5° to 1.5° than the inclination angle (θ13a, θ13b) of the third and fourth inclined surfaces (13a, 13b) defining the concave fastening surface. an annular or circular boss-side fastening surface formed on an end surface of a rigid boss in the flexible externally toothed gear; a shaft-side fastening surface formed on an end surface of the shaft member to be fastened to fix to the boss coaxially; and a fastener for fixedly fastening the boss and the shaft member along a direction of a center axis line in a state in which the boss-side fastening surface is in coaxial contact with the shaft-side fastening surface, characterized in that one fastening surface (12, 13) which is either one of the boss-side fastening surface and the shaft-side fastening surface is a convex fastening surface defined by first and second inclined surfaces (12a, 12b), the first and second inclined surfaces (12a, 12b) being intersected at a prescribed angle (θ12a, θ12b) to form a ridge line along a diameter of the one fastening surface (12, 13), and the first and second inclined surfaces (12a, 12b) being symmetrical with each other with respect to a plane including the ridge line and the center axis line; the other fastening surface (13, 12) of the boss-side fastening surface and the shaft-side fastening surface is a concave fastening surface defined by third and fourth inclined surfaces (13a, 13b), the third and fourth inclined surfaces (13a, 13b) being intersected at a prescribed angle to form a trough line along a diameter of the other fastening surface (13, 12), and the third and fourth inclined surfaces (13a, 13b) being symmetrical with each other with respect to a flat plane including the trough line and the center axis line; and an inclination angle (θ13a, θ13b) of each of the first to fourth inclined surfaces (13a, 13b) with respect to an axis-orthogonal plane has a value in a range between 2° and 16°, where the axis-orthogonal plane is a plane orthogonal to the center axis line; and the first and second inclined surfaces (12a, 12b) defining the convex fastening surface have an inclination angle (θ12a, θ12b) that is smaller by an amount ranging from 0.5° to 1.5° than the inclination angle (θ13a, θ13b) of the third and fourth inclined surfaces (13a, 13b) defining the concave fastening surface.