A mechanical sealing device comprising: a stationary slide ring adapted to be hermetically fixed in a housing; a rotary slide ring which slides and closely contacts with said stationary slide ring, the rotary slide ring comprising an inner bore, the rotary slide ring being adapted to be rotatable with a rotary shaft when inserted into the inner bore; and an O-ring housed in in an annular ramp portion of an inner bore of said rotary slide ring; wherein said rotary slide ring is positionable on an outer circumference surface of said rotary shaft to be able to move in the axial direction of said rotary shaft via said O-ring; and characterized in that: said mechanical sealing device further comprises an annular ring member forming a triangle shape in axial section and housed in the annular ramp portion of the inner bore of the rotary slide ring as adjoining the O-ring at an atmosphere side, the ring member being spaced to an outer circumference surface of the rotary shaft; said annular ramp portion of said inner bore of said rotary slide ring comprises an inner circumference face tapered to become gradually narrower from a high pressure environment side toward an atmosphere side; said ring member having a relatively lower gas permeability than that of said O-ring and comprises a tapered outer circumference face of which an outer diameter becomes smaller from the high pressure environment side towards the atmosphere side so as to correspond to the tapered inner circumference face of said annular ramp portion of said inner bore of said rotary slide ring, and an inner circumference face in which an annular projected portion is provided at an end portion adjacent to said O-ring in a cross-section along an axial direction of said annular ring member, said annular projected portion projecting radially inward; said tapered outer circumference face of said ring member and said tapered inner circumference face of said rotary slide ring contact each other; and an expression D 1 < D 2 < D 3 is satisfied in which D1 is a diameter of a portion of said rotary shaft where said ring member is positioned, D2 is an innermost diameter of said ring member, and D3 is an inner diameter of a portion of said rotary slide ring where said portion at said atmosphere side from said ring member in the inner bore of said rotary slide ring is positioned. a stationary slide ring adapted to be hermetically fixed in a housing; a rotary slide ring which slides and closely contacts with said stationary slide ring, the rotary slide ring comprising an inner bore, the rotary slide ring being adapted to be rotatable with a rotary shaft when inserted into the inner bore; and an O-ring housed in in an annular ramp portion of an inner bore of said rotary slide ring; wherein said rotary slide ring is positionable on an outer circumference surface of said rotary shaft to be able to move in the axial direction of said rotary shaft via said O-ring; said mechanical sealing device further comprises an annular ring member forming a triangle shape in axial section and housed in the annular ramp portion of the inner bore of the rotary slide ring as adjoining the O-ring at an atmosphere side, the ring member being spaced to an outer circumference surface of the rotary shaft; said annular ramp portion of said inner bore of said rotary slide ring comprises an inner circumference face tapered to become gradually narrower from a high pressure environment side toward an atmosphere side; said ring member having a relatively lower gas permeability than that of said O-ring and comprises a tapered outer circumference face of which an outer diameter becomes smaller from the high pressure environment side towards the atmosphere side so as to correspond to the tapered inner circumference face of said annular ramp portion of said inner bore of said rotary slide ring, and an inner circumference face in which an annular projected portion is provided at an end portion adjacent to said O-ring in a cross-section along an axial direction of said annular ring member, said annular projected portion projecting radially inward; said tapered outer circumference face of said ring member and said tapered inner circumference face of said rotary slide ring contact each other; and an expression D 1 < D 2 < D 3 is satisfied in which D1 is a diameter of a portion of said rotary shaft where said ring member is positioned, D2 is an innermost diameter of said ring member, and D3 is an inner diameter of a portion of said rotary slide ring where said portion at said atmosphere side from said ring member in the inner bore of said rotary slide ring is positioned.