A mechanical sealing device according to an embodiment of the present invention includes: a sealing ring (13) attached to a protrusion (11a) protruding from a housing (11); a mating ring (23) attached to an end portion of a rotary shaft (21); a welded bellows provided around the protrusion (11a) and located between the housing (11) and the sealing ring (13). The welded bellows (15) presses the sealing ring (13) in an axial direction to cause the sealing ring (13) to be in contact with the mating ring (23), thereby ensuring high pressure force, and in particular, uniform pressure force along its entire circumference, so as to achieve high liquid tightness, while maintaining the sizes in axial and radial directions small. Furthermore, the welded bellows (15) works as a sealing member of blocking a liquid leaked out through the O-ring (14), thereby achieving assured liquid tightness. Accordingly, an outside-type mechanical sealing device can be provided which has a compact size, and is capable of ensuring high liquid tightness in response to a high rotation speed and a high liquid pressure.