In the previous report (Tetsu-to-Hagané, 47 (1961) No. 3, p. 390), it was revealed that the cooling of a billet in the mold was largely reduced by formation of the air gap between the billet and the mold in the lower part of the mold.In this report, the result of experiments which were held in order to search the commencement of formation of the air gap and the thickness of the air gap between the billet and the mold shall be revealed.The air gap between the billet and the mold occurred when the contraction of the outer shell of the billet by cooling became greater than the creep of the outer shell of the billet by the ferrostatic pressure of the molten steel which was retained yet in the core of the billet.The contraction of the outer shell of the billet could be calculated by the temperature change of the shell. On the other hand, the creep of the shell could be calculated by the thickness and the temperature change of the shell and the ferrostatic pressure of the molten steel which was retained in the core of the billet. Experiments were carried to search the thickness of the shell by means of the pour-out test and to search the temperature change of the shell by means of inserting the thermocouple into the shell of the billet.The experimental results were as follows. The commencement of the air gap formation was at 250mm below the meniscus. The taper which occurred between the billet and the mold was found to be 1·66/1000-3·15/1000.These figures about the air gap between the billet and the mold should have a great meaning n analysing the reasonable cooling of the billet in the mold in the following report.