A mechanical seal device comprising a primary-side mechanical seal having a first mating ring that is rotatable with a rotating shaft, as well as an opposing first seal ring that slidably contacts the first mating ring and a secondary-side mechanical seal having a second mating ring that is rotatable with the rotating shaft, as well as an opposing second seal ring that slidably contacts the second mating ring, so as to seal between the rotating shaft and a seal cover fixed to an equipment main body; the seal cover being a split structure comprising at least a first seal cover (107, 108) and a second seal cover, the first seal cover (107, 108) is detachably connected in an axial direction to the second seal cover, the first seal cover (107, 108) being a split structure comprising an outer-diameter-side seal cover and an inner-diameter-side seal cover, the inner-diameter-side seal cover being detachably connected to the outer-diameter-side seal cover from an outer side in the axial direction by a coupling bolt, the second seal ring being connected to the inner-diameter-side seal cover in a securely following manner, the outer-diameter-side seal cover having a circular cutout on an atmosphere inner diameter side, the inner-diameter-side seal cover having a circular projection that projects toward an outer diameter side and has a through hole, the circular projection of the inner-diameter-side seal cover being aligned in position with the circular cutout of the outer-diameter-side seal cover, the coupling bolt is screwed to the outer-diameter-side seal cover through the through hole of the circular projection of the inner-diameter-side seal cover, wherein the seal ring is held by a retainer, an adapter is welded and fixed to the inner-diameter-side cover, the retainer is welded and fixed to one end of a bellows, the adapter is welded and fixed to the other end of the bellows, and a fixed-side seal assembly constituted by the seal ring, the retainer, the bellows, and the adapter is connected to the inner-diameter-side seal cover in a securely following manner, wherein the mechanical seal device further comprises a sleeve fixed to the rotating shaft and provided with a circular projection extending in the axial direction, the first mating ring is closely fitted and fixed to a first end of the circular projection, characterized in that the second mating ring is closely fitted and fixed to a second end of the circular projection opposite the first end in the axial direction. first seal ring that slidably contacts the first mating ring and a secondary-side mechanical seal having a second mating ring that is rotatable with the rotating shaft, as well as an opposing second seal ring that slidably contacts the second mating ring, so as to seal between the rotating shaft and a seal cover fixed to an equipment main body; the seal cover being a split structure comprising at least a first seal cover (107, 108) and a second seal cover, the first seal cover (107, 108) is detachably connected in an axial direction to the second seal cover, the first seal cover (107, 108) being a split structure comprising an outer-diameter-side seal cover and an inner-diameter-side seal cover, the inner-diameter-side seal cover being detachably connected to the outer-diameter-side seal cover from an outer side in the axial direction by a coupling bolt, the second seal ring being connected to the inner-diameter-side seal cover in a securely following manner, the outer-diameter-side seal cover having a circular cutout on an atmosphere inner diameter side, the inner-diameter-side seal cover having a circular projection that projects toward an outer diameter side and has a through hole, the circular projection of the inner-diameter-side seal cover being aligned in position with the circular cutout of the outer-diameter-side seal cover, the coupling bolt is screwed to the outer-diameter-side seal cover through the through hole of the circular projection of the inner-diameter-side seal cover, wherein the seal ring is held by a retainer, an adapter is welded and fixed to the inner-diameter-side cover, the retainer is welded and fixed to one end of a bellows, the adapter is welded and fixed to the other end of the bellows, and a fixed-side seal assembly constituted by the seal ring, the retainer, the bellows, and the adapter is connected to the inner-diameter-side seal cover in a securely following manner, wherein the mechanical seal device further comprises a sleeve fixed to the rotating shaft and provided with a circular projection extending in the axial direction, the first mating ring is closely fitted and fixed to a first end of the circular projection,