A mechanical seal device comprising: a rotary ring provided on a rotary shaft so as to integrally rotate with the rotary shaft, and on which a rotary ring side sliding face that constitutes one face of dynamic seal surfaces, which dynamically seals between a first fluid within the apparatus and a second fluid provided closer to outside the apparatus than the first fluid, is formed; a stationary ring assembly on which a stationary ring side sliding face that slides with the rotary ring side sliding face and constitutes the other surface of the dynamic seal surface is formed; a seal cover which contains the stationary ring assembly on an inner diameter side and supports the stationary ring assembly to move freely in an axial direction of the rotary shaft; a first O-ring which is sandwiched by the seal cover and the stationary ring assembly in a radial direction of the rotary shaft to seal the first fluid; and a second O-ring which is provided closer to outside the apparatus than the first O-ring, and which is sandwiched by the seal cover and the stationary ring assembly in the radial direction, to seal the second fluid, characterized by: a low-pressure chamber, which is sealed from the first fluid and the second fluid by the first O-ring and the second O-ring and which maintains continual pressure lower than the pressure of the first fluid and that of the second fluid, is formed between the first O-ring and the second O-ring; a first pressure acting diameter, formed by a boundary of a pressure acting region, in which the first fluid presses the stationary ring assembly except the stationary ring side sliding face, and a pressure not acting region, in which the first fluid does not press the stationary ring assembly, is located, when viewed in the radial direction, on a second seal end side which is an end of the dynamic seal surface at the second fluid side with respect to a first seal end which is an end of the dynamic seal surfaces at the first fluid side; and a second pressure acting diameter, formed by a boundary of a pressure acting region, in which the second fluid presses the stationary ring assembly except the stationary ring side sliding face, and a pressure not acting region, in which the second fluid does not press the stationary ring assembly, is located, when viewed in the radial direction, on the first seal end side with respect to the second seal end. a rotary ring provided on a rotary shaft so as to integrally rotate with the rotary shaft, and on which a rotary ring side sliding face that constitutes one face of dynamic seal surfaces, which dynamically seals between a first fluid within the apparatus and a second fluid provided closer to outside the apparatus than the first fluid, is formed; a stationary ring assembly on which a stationary ring side sliding face that slides with the rotary ring side sliding face and constitutes the other surface of the dynamic seal surface is formed; a seal cover which contains the stationary ring assembly on an inner diameter side and supports the stationary ring assembly to move freely in an axial direction of the rotary shaft; a first O-ring which is sandwiched by the seal cover and the stationary ring assembly in a radial direction of the rotary shaft to seal the first fluid; and a second O-ring which is provided closer to outside the apparatus than the first O-ring, and which is sandwiched by the seal cover and the stationary ring assembly in the radial direction, to seal the second fluid, a low-pressure chamber, which is sealed from the first fluid and the second fluid by the first O-ring and the second O-ring and which maintains continual pressure lower than the pressure of the first fluid and that of the second fluid, is formed between the first O-ring and the second O-ring; a first pressure acting diameter, formed by a boundary of a pressure acting region, in which the first fluid presses the stationary ring assembly except the stationary ring side sliding face, and a pressure not acting region, in which the first fluid does not press the stationary ring assembly, is located, when viewed in the radial direction, on a second seal end side which is an end of the dynamic seal surface at the second fluid side with respect to a first seal end which is an end of the dynamic seal surfaces at the first fluid side; and a second pressure acting diameter, formed by a boundary of a pressure acting region, in which the second fluid presses the stationary ring assembly except the stationary ring side sliding face, and a pressure not acting region, in which the second fluid does not press the stationary ring assembly, is located, when viewed in the radial direction, on the first seal end side with respect to the second seal end.