As mentioned above, "How to obtain the sound ingots" is the first, most important technical problem than any others in high quality special steel manufacturing. And the hydrogen-boiling in the ingot of the low C-special steel melting is one of the most troublesome problems, especially in the humid season. We suffered formerly about this problem for a long time too. So that here I introduced the considerations on our bitter experiences, for reference. I showed at first, many statistics about [H]_Tap which we measured and then the test results in our practical melting operations. Finally I summarize the instructions, how to suppress the increase of [H] and to avoid the hydrogen-boiling in the low C-special steel. (1) About the increase of [H] from the oxidizing period to the reduction stage. In order to keep [H] (in the connection with [O]) in the lower level at the earlier stage of reduction, it is necessary to keep [H]²×[O] value at the end of oxidizing period as smaller as possible, and to keep Δ[H](=k₁[H]²×Δ[C]＋k₂) as higher as possible. For these purposes, the most important point is, after all, how to choose the optimum temperature to begin the oxygen-blowing and at the same time its finishing temperature. The effective conditions for decarburization are also effective for dehydrogenation. Especially in this case, we must pay attention to the fact that, in the earlier stage of reduction, [H] is apt to increase to higher amount in low C steels than in medium, high C steels, even when the values of [H]²×[O] at the end of the oxidizing period are same. (2) About the degree of slag-off at the end of oxidizing period: as completely(by another reason) and as rapid as possible. About 1m exposure of bath surface may be sufficient. (3) After slag-off of the oxidizing period, we can say there is no difference in the charging sequence, that is, at first deoxidizing agent, or at first reducing slag. But we can say that [H]_(Tap) in the case when we use 50% Ca CO₃is lower than that of 100% CaO in the humid season. (4) The limes must be perfectly dried. It must be aimed at least H₂O<0.5%. About the ferro-alloys (especially the ferro-Cr) and other charging materials (including the scraps) too. (5) Reducing period must be operated as quick as possible. Basicity of reducing slag must be at least>2.0 and is better to be kept somewhat higher, as nearer to the end of reducing period. Of course, never sudden change of basicity. Slag conditions must be kept in the state of A or B, for the hydrogen problem in the humid season. And for this purpose the important point is the temperature control, that is, keeping the temperature in only gradually progressive drop without specially strong heating in the way from the end of the oxygen-blowing to the tapping. (6) Even though with the full cautions mentioned above, if H_(Tap) increases over the limited value, we can suppress such increasing amount of [H] by the argon-blowing max. 1.5㎥/Ton of steel, and avoid surely the hydrogen-boiling. Many practical data in this report are those which were measured formerly by our "Hydrogen Problem Committee" of Japan Special Steel Co. This time, I could introduce here these data, under permission of Japan Special Steel Co. I thank for that permission. At last for your eager listening, I want to express the thankfulness.