A numerical computation of the secular variation of the production rate of atmospheric radiocarbon was carried using the secular variation of atmospheric radiocarbon concentration. In order to show the relation between atmospheric radiocarbon concentration and geomagnetism, the computed secular variation in radiocarbon production rate was converted to that of the geomagnetic moment, assuming a simple dependence of the production rate on the geomagnetic moment. The results of these computations of secular variation of the geomagnetic moment showed an essential discrepancy from the observed values in the period of de Vries's peak. This discrepancy shows that the simple dependence of the radiocarbon production rate on the geomagnetic moment does not hold in the computation of the secular variation of paleogeomagnetic moment from radiocarbon production rates. The results of this study indicate that the variation of the radiocarbon production rate depends not only on the variations in paleogeomagnetisms, but also upon the modulation of the magnetosphere by solar activities.