A mechanical seal device in which, between a processing equipment which performs processing under a sterile condition and a rotary shaft used with its one end inserted in the processing equipment from an atmosphere side, seal parts are disposed outside the processing equipment at least at two places along a length direction of the rotary shaft from the processing equipment side toward the atmosphere side seals the rotary shaft, and when the inside of the processing equipment and the seal parts are sterilized, the atmosphere-side seal part is also easily and surely sterilized similarly to the other seal part. An airtight area is provided on an atmosphere side of an atmosphere-side seal part so that an area communicating with the atmosphere-side seal part is kept airtight all along a circumferential direction of a rotary shaft, and sterilizing gas supplied to a sealing fluid supplied area, which is an area between the rotary shaft and a device main body of the mechanical seal device, in order to sterilize the seal parts is retained in the airtight area.