In this paper the method of natural orthogonal components (NOC) is utilized to decompose the daily magnetic variation, expressing the variation as a summation of the eigenmodes. The results show that the first and second NOC eigenmodes represent the solar quiet daily variation, Sq, and the disturbance-daily variation, SD, respectively. The third and fourth eigenmodes may be related to specific currents in the magnetosphere. Remarkable day-to-day (DTD) variations exist in the amplitudes of the eigenmodes. The combination of the eigenmodes reproduces the complicated characteristics in the DTD variation of the daily magnetic variation. The decomposition of the daily magnetic variation presented in this paper is helpful for understanding the mechanism of the daily variation and its DTD variation. In addition, this decomposition method provides a ground-based monitoring of the current systems in several important regions of the near-Earth space.