A mechanical seal device for sealing a sealed fluid inside of a machine at a space formed between a device main body and a rotary shaft, comprising: a seal case adapted to be attached to the device main body, a rotary seal ring adapted to be attached to the rotary shaft, and, a stationary seal ring arranged in an axial direction opposing to the rotary seal ring, a bellows attached extendably in the axial direction between the stationary seal ring and the seal case, a resilient member pressing the stationary seal ring toward the rotary seal ring, a static pressure fluid supplying hole, formed on the seal case, for supplying a static pressure fluid to a static pressure fluid chamber arranged at an inner circumferential side of the bellows, a communication hole, for guiding the static pressure fluid of the static pressure fluid chamber to sealing surfaces (55, 62) between the stationary seal ring and the rotary seal ring, is formed on the stationary seal ring, an annular groove connecting to the communication hole is formed on the sealing surface of the stationary seal ring, and the mechanical seal device is adapted to have the sealed fluid is arranged at an outer circumferential side of the bellows, characterised in that the bellows applies compressive force pressing the stationary seal ring toward the rotary seal ring, an O-ring is arranged between the seal case and the stationary seal ring at the inner circumferential side of the bellows, holding the stationary seal ring movably in the axial direction and sealing the static pressure fluid chamber from an atmosphere side space, an outer diameter (d bb) of the annular groove is designed so that it is nearly equal to a pressure acting diameter (d b) of the bellows, and an inner diameter (d ob) of the annular groove is designed so that it is nearly equal to a contact diameter (d o) where the O-ring contacts the stationary seal ring. a seal case adapted to be attached to the device main body, a rotary seal ring adapted to be attached to the rotary shaft, and, a stationary seal ring arranged in an axial direction opposing to the rotary seal ring, a bellows attached extendably in the axial direction between the stationary seal ring and the seal case, a resilient member pressing the stationary seal ring toward the rotary seal ring, a static pressure fluid supplying hole, formed on the seal case, for supplying a static pressure fluid to a static pressure fluid chamber arranged at an inner circumferential side of the bellows, a communication hole, for guiding the static pressure fluid of the static pressure fluid chamber to sealing surfaces (55, 62) between the stationary seal ring and the rotary seal ring, is formed on the stationary seal ring, an annular groove connecting to the communication hole is formed on the sealing surface of the stationary seal ring, and the mechanical seal device is adapted to have the sealed fluid is arranged at an outer circumferential side of the bellows, characterised in that the bellows applies compressive force pressing the stationary seal ring toward the rotary seal ring, an O-ring is arranged between the seal case and the stationary seal ring at the inner circumferential side of the bellows, holding the stationary seal ring movably in the axial direction and sealing the static pressure fluid chamber from an atmosphere side space, an outer diameter (d bb) of the annular groove is designed so that it is nearly equal to a pressure acting diameter (d b) of the bellows, and an inner diameter (d ob) of the annular groove is designed so that it is nearly equal to a contact diameter (d o) where the O-ring contacts the stationary seal ring.