A mechanical seal device comprising a seal cover mounted to an axial outer surface of a apparatus body having an inner circumference into which a rotary shaft is inserted, a first seal ring axially-movably arranged in an inner circumference side of said seal cover, a second seal ring fixed to said rotary shaft, a spring arranged in the inner circumference side of said seal cover, pressing said first seal ring in a direction toward said second seal ring, a bellows mounted to the inner circumference side of said seal cover and axially-expandably mounted between said seal cover and said first seal ring, wherein: said spring is mounted to said seal cover along the axial direction in which said bellows is expanded and contracted; said bellows comprises a metallic main bellows body and a pair of metallic bellows end holding portions (5B, 5C) joined respectively with both axial ends of the main bellows body; one of said bellows end holding portions connected to said first seal ring is formed with a projection portion extending outwardly in a radial direction; said spring presses said first seal ring via said projection portion to a direction of said second seal ring; characterized in that a gap is formed between the bellows and the outer circumference surface of the rotary shaft, and the size of the gap is calculated to allow flow without solidifying the seal fluid to clog, a center of the bellows in the radial direction is within the range of the width (γ) of a sliding surface in the radial direction, the sliding surface corresponding to the first sealing ring and being sealingly contacted with a sliding surface corresponding to the second sealing ring, and the pressing force to the sliding surfaces (3A, 10A) is defined by not only the pressing force of the spring, but also the pressing force of the sealed fluid in the width (δ) of the radial direction between a joint portion of the main bellows body with the bellows end holding portion in the radial direction and an inner circumference surface of a projection portion corresponding to the first sealing ring and the sliding surface, said bellows is mounted so that said bellows maintains an approximate free length after being mounted; and number of peaks of said main bellows body is 3 to 5. said spring is mounted to said seal cover along the axial direction in which said bellows is expanded and contracted; said bellows comprises a metallic main bellows body and a pair of metallic bellows end holding portions (5B, 5C) joined respectively with both axial ends of the main bellows body; one of said bellows end holding portions connected to said first seal ring is formed with a projection portion extending outwardly in a radial direction; said spring presses said first seal ring via said projection portion to a direction of said second seal ring; characterized in that a gap is formed between the bellows and the outer circumference surface of the rotary shaft, and the size of the gap is calculated to allow flow without solidifying the seal fluid to clog, a center of the bellows in the radial direction is within the range of the width (γ) of a sliding surface in the radial direction, the sliding surface corresponding to the first sealing ring and being sealingly contacted with a sliding surface corresponding to the second sealing ring, and the pressing force to the sliding surfaces (3A, 10A) is defined by not only the pressing force of the spring, but also the pressing force of the sealed fluid in the width (δ) of the radial direction between a joint portion of the main bellows body with the bellows end holding portion in the radial direction and an inner circumference surface of a projection portion corresponding to the first sealing ring and the sliding surface, said bellows is mounted so that said bellows maintains an approximate free length after being mounted; and number of peaks of said main bellows body is 3 to 5.