A bellows mechanical seal comprising: a seal ring; a collar (3, 3A, 3B, 3C, 3D) attached to the seal ring through a bellows; a mating ring slidably contacting the seal ring; and a secondary seal (60, 80) configured to seal a gap between the collar (3, 3A, 3B, 3C, 3D) and a rotary shaft or between the collar (3, 3A, 3B, 3C, 3D) and a housing, wherein the collar (3, 3A, 3B, 3C, 3D) has, at a first end thereof in an axial direction, a restriction piece (4, 4c, 4d, 8, 8c, 8d) extending toward at least one of an inner diameter side and an outer diameter side and, at a second end thereof in the axial direction, threaded screw holes extending in the axial direction, the bellows mechanical seal further includes an adaptor (50, 56, 70, 76) which is attached to the collar (3, 3A, 3B, 3C, 3D), whose axial movement is restricted by the restriction piece (4, 4c, 4d, 8, 8c, 8d), and which extends to the inner diameter side or the outer diameter side over the restriction piece (4, 4c, 4d, 8, 8c, 8d), the collar (3, 3A, 3B, 3C, 3D) and the adaptor (50, 56, 70, 76) define at least part of a housing space (S, S') having the secondary seal (60, 80) housed therein, characterized in that a thickness as the axial length of the secondary seal (60, 80) is slightly greater than the axial length of the housing space (S, S'), and a radial width as the radial length of the secondary seal (60, 80) is slightly smaller than the radial length of the housing space (S, S'), the collar (3, 3A, 3B, 3C, 3D) is slightly compressed in the axial direction while slightly expanding in the radial direction, and is directly or indirectly fixed to the housing or the rotary shaft in the axial direction by tightening set screws (16, 25) in a state in which the adaptor (50, 56, 70, 76) and the secondary seal (60, 80) are attached to an outer diameter side or an inner diameter side of the collar (3, 3A, 3B, 3C, 3D). a seal ring; a collar (3, 3A, 3B, 3C, 3D) attached to the seal ring through a bellows; a mating ring slidably contacting the seal ring; and a secondary seal (60, 80) configured to seal a gap between the collar (3, 3A, 3B, 3C, 3D) and a rotary shaft or between the collar (3, 3A, 3B, 3C, 3D) and a housing, wherein the collar (3, 3A, 3B, 3C, 3D) has, at a first end thereof in an axial direction, a restriction piece (4, 4c, 4d, 8, 8c, 8d) extending toward at least one of an inner diameter side and an outer diameter side and, at a second end thereof in the axial direction, threaded screw holes extending in the axial direction, the bellows mechanical seal further includes an adaptor (50, 56, 70, 76) which is attached to the collar (3, 3A, 3B, 3C, 3D), whose axial movement is restricted by the restriction piece (4, 4c, 4d, 8, 8c, 8d), and which extends to the inner diameter side or the outer diameter side over the restriction piece (4, 4c, 4d, 8, 8c, 8d), the collar (3, 3A, 3B, 3C, 3D) and the adaptor (50, 56, 70, 76) define at least part of a housing space (S, S') having the secondary seal (60, 80) housed therein, characterized in that a thickness as the axial length of the secondary seal (60, 80) is slightly greater than the axial length of the housing space (S, S'), and a radial width as the radial length of the secondary seal (60, 80) is slightly smaller than the radial length of the housing space (S, S'), the collar (3, 3A, 3B, 3C, 3D) is slightly compressed in the axial direction while slightly expanding in the radial direction, and is directly or indirectly fixed to the housing or the rotary shaft in the axial direction by tightening set screws (16, 25) in a state in which the adaptor (50, 56, 70, 76) and the secondary seal (60, 80) are attached to an outer diameter side or an inner diameter side of the collar (3, 3A, 3B, 3C, 3D).