To surely prevent intrusion of air from a connecting part in a connection structure for connecting a circulation tube and an immersion tube which are projectingly provided at the bottom of a vacuum degassing tank only by slightly modifying a conventional structure.

A second refractory material 5 lined on the immersion tube 1 has an upper end surface formed in a step-like shape with the higher portion on the inner peripheral side being a projection 22. A first refractory material 11 assembled to the circulation tube 2 has a lower end surface formed in a step-like shape substantially matched to the step with the lower portion on the inner peripheral side being a recess 23 to be fitted to the projection 22. When the immersion tube 1 is connected, the projection 22 is fitted into the recess 23. The air to intrude from the outside is blocked by the projection 22 to prevent an increase in nitrogen concentration due to mixing of air into molten steel.