A mechanical seal device comprising: a mating ring having an axially perpendicular sealing face and having a flange projecting in a radial direction; an axially movable seal ring having an axially perpendicular sealing face to be brought into contact with the sealing face of the mating ring; a resilient member that presses the seal ring to a side of the mating ring; and a case inside which the seal ring and the resilient member are accommodated, wherein a fluid is sealed by bringing the axially perpendicular sealing faces (30a, 41a) of both the rings into contact with each other and relatively rotating the sealing faces (30a, 41a); and characterized in that the case has guide pieces projecting on the side of the mating ring in the axial direction; the seal ring has cutouts on an outer face; and by engaging the guide pieces with the cutouts, the seal ring is incapable of being relatively rotated with respect to the case, wherein the guide pieces each have an engagement portion projecting in the radial direction, and are resiliently held by engaging the engagement portions with the flange of the mating ring, and wherein in the flange, cutouts having such widths that the engagement portions are capable of passing through are formed in a circumferential direction of the flange, and wherein the seal ring and the mating ring are resiliently held by a) matching the guide pieces with positions of the cutouts of the mating ring, b) pushing the guide pieces into the axial direction against a resilient force of the resilient member, c) rotating the case in the circumferential direction from a position where the engagement portions exceed the flange, and d) engaging the engagement portions and the flange with each other. a mating ring having an axially perpendicular sealing face and having a flange projecting in a radial direction; an axially movable seal ring having an axially perpendicular sealing face to be brought into contact with the sealing face of the mating ring; a resilient member that presses the seal ring to a side of the mating ring; and a case inside which the seal ring and the resilient member are accommodated, wherein a fluid is sealed by bringing the axially perpendicular sealing faces (30a, 41a) of both the rings into contact with each other and relatively rotating the sealing faces (30a, 41a); and characterized in that the case has guide pieces projecting on the side of the mating ring in the axial direction; the seal ring has cutouts on an outer face; and by engaging the guide pieces with the cutouts, the seal ring is incapable of being relatively rotated with respect to the case, wherein the guide pieces each have an engagement portion projecting in the radial direction, and are resiliently held by engaging the engagement portions with the flange of the mating ring, and wherein in the flange, cutouts having such widths that the engagement portions are capable of passing through are formed in a circumferential direction of the flange, and wherein the seal ring and the mating ring are resiliently held by a) matching the guide pieces with positions of the cutouts of the mating ring, b) pushing the guide pieces into the axial direction against a resilient force of the resilient member, c) rotating the case in the circumferential direction from a position where the engagement portions exceed the flange, and d) engaging the engagement portions and the flange with each other. a) matching the guide pieces with positions of the cutouts of the mating ring, b) pushing the guide pieces into the axial direction against a resilient force of the resilient member, c) rotating the case in the circumferential direction from a position where the engagement portions exceed the flange, and d) engaging the engagement portions and the flange with each other.