A mechanical sealing device (1, 2) provided between a housing and a rotary shaft, the mechanical sealing device (1, 2) being suitable to prevent a liquid from leaking out from a connection portion of a first passage and a second passage when, in use, a liquid is supplied from the first passage to the second passage or in the reverse direction, wherein the housing has a protrusion protruding in a substantially columnar shape, and the first passage is formed inside the protrusion in an axial direction of the protrusion, and wherein the second passage is formed inside the rotary shaft to extend in the axial direction along which the first passage extends, the mechanical sealing device comprising: a) a fixed-side annular portion unrotatably and slightly movably in the axial direction attached to the housing, the fixed-side annular portion being sealed to the protrusion of the housing with a secondary sealing member; b) a rotation-side annular portion attached to the rotary shaft so as to be integrally rotatable with the rotary shaft, the rotation-side annular portion being sealed to the rotary shaft with a rotation-side sealing member; and c) a bellows provided around the protrusion so as to be positioned between the housing and the fixed-side annular portion, configured to press the fixed-side annular portion toward the rotation-side annular portion to cause the fixed-side annular portion to be in contact with the rotation-side annular portion; wherein the secondary sealing member is disposed between a contact portion of the bellows with the housing and the connection portion characterized in that the secondary sealing member is an O-ring and seals a gap between the fixed-side annular portion and the protrusion into which the liquid leaking out from the connection portion flows. the mechanical sealing device comprising: a) a fixed-side annular portion unrotatably and slightly movably in the axial direction attached to the housing, the fixed-side annular portion being sealed to the protrusion of the housing with a secondary sealing member; b) a rotation-side annular portion attached to the rotary shaft so as to be integrally rotatable with the rotary shaft, the rotation-side annular portion being sealed to the rotary shaft with a rotation-side sealing member; and c) a bellows provided around the protrusion so as to be positioned between the housing and the fixed-side annular portion, configured to press the fixed-side annular portion toward the rotation-side annular portion to cause the fixed-side annular portion to be in contact with the rotation-side annular portion; a) a fixed-side annular portion unrotatably and slightly movably in the axial direction attached to the housing, the fixed-side annular portion being sealed to the protrusion of the housing with a secondary sealing member; b) a rotation-side annular portion attached to the rotary shaft so as to be integrally rotatable with the rotary shaft, the rotation-side annular portion being sealed to the rotary shaft with a rotation-side sealing member; and c) a bellows provided around the protrusion so as to be positioned between the housing and the fixed-side annular portion, configured to press the fixed-side annular portion toward the rotation-side annular portion to cause the fixed-side annular portion to be in contact with the rotation-side annular portion; wherein the secondary sealing member is disposed between a contact portion of the bellows with the housing and the connection portion characterized in that the secondary sealing member is an O-ring and seals a gap between the fixed-side annular portion and the protrusion into which the liquid leaking out from the connection portion flows.