Using the Km index, the F10.7 index, and solar wind data from 1965 to 1996, we have examined the variations for the efficiency of solar wind--magnetosphere--ionosphere (SMI) coupling. It is found that the efficiency of SMI coupling depends on the total solar zenith angle, which is defined by the sum of cosine of the solar zenith angles at the northern and southern corrected geomagnetic (CGM) poles. This result suggests that the diurnal and semiannual variations of the coupling efficiency are explained by the variations of the total solar zenith angle. Further, we find that the coupling efficiency during the period of high solar activity (F10.7) tends to be lower than that during the period of low solar activity. This suggests that the efficiency of SMI coupling has a dependence on the solar activity. Our interpretation of these characteristics is that the total Pedersen conductivity on both northern and southern polar caps controls the efficiency of SMI coupling. We find that the variations of the coupling efficiency can be reproduced when our empirical conductivity model is applied in the Siscoe-Hill model. This suggests that the ionospheric conductivity plays a significant role in SMI coupling and produces diurnal, semiannual, and solar cycle variations of geomagnetic activity.