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 of both the rings into contact with each other and relatively rotating the sealing faces ; 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 of both the rings into contact with each other and relatively rotating the sealing faces ; 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. <EOS>