Provide a sealing device with which simple and reliable operation can be performed. When stored, a liquid agent container, in which a sealing agent is contained, is temporarily held above a injection unit. When the liquid agent container is forcibly pressed downward in use, the upper end of a pressing jig penetrates through an aluminum seal into the container, and the sealing agent flows out to a compressor unit is activated and compressed air is supplied into the liquid agent container to place the pressurized sealing agent into a pneumatic tire through a joint hose.