(1) If the constancy of IQ is understood physiologically, it must mean the constancy of a particular developmental ratio of the individual against the average development of the standardizing group. Namely, we are to read the IQ formula as, [numerical formula] =Developmental ratio of the individual × 100 (2) In case of IQ of young children, the ordinary formula of IQ has a disadvantage, when considered as such. For example, a developmental ratio of a child, whose C. A. is 2, and whose M. A. is 3, accordingly whose IQ is 150, is not 1.5, because we should move the original point of the formula from a birth to a conception, if we wish to get a strict ratio. If we do so, we get the developmental ratio of 1.35 (12×3+10 divided by 12×2+10) instead of 1.5. For maturely born children, the corrected formula of IQ in general, is, [numerical formula] (3) The effect of correction is large for very young children, but negligible for children of more than 10 years old. Tab. 1 and Fig. 1 show errorneous IQs calculated by the ordinary formula in terms of developmental ratio 1.5~0.5 and C. A. 1~10. (the value of k developmental ratio and n C. A. is ((12×n+10)×k-10)/(12×n), and the error is ((12×n+10)×k-10)/(12×n)-k×100) (4) For prematurely born children, the corrected formula is, [numerical formula] (t being the period of pregnancy of the child) For example, IQ of a child whose C. A. is 3, and whose M. A. is 2 years and 10 months, is 100, (12×2+10+10)/(12×3+8), if he is born at 8th month, instead of 94 which is given by the ordinary formula. A few reports on the intelligence of prematurely born children that they had inferior IQ's were reexamined and the data were re-calculated by the correction formula (3). wefound that, in general, they were not inferior, but quite normal. They say that prematurely born children are inferior, when they are very young. But we do not necessary hear this, when they enter into teenages. It seems to us they are evaluated by undue C. A. when young, but their true developmental ratio appear soon or later.