Abstract: A mechanical seal made in a unit construction in which a seal unit case is fitted into and held by the inner periphery of a seal housing in a non-rotatable but axially slidable manner, the seal housing houses a rotary ring disposed on a rotary shaft in a non-rotatable but axially slidable manner, and a stationary ring in contact with the rotary ring at the sealing end surfaces thereof is securely fixed to the seal housing. The mechanical seal can be automatically moved to a position where a first fluid and a second fluid are balanced in pressure in response to a difference in pressure between these two fluids.