Long-term variations of energetic particles in the radiation belts were examined using data from the NOAA (1979--2003) satellites. A significant flux variation with solar cycle was detected together with both semiannual and recurrent flux variations. It was revealed that the phase of flux variations for the solar cycle depends on both particle energy and distance from the Earth; the outer belt shifted inward during the solar active period and outward during the solar quiet period. The numerical simulation for radial diffusion reproduced the flux variation and the outer belt shift qualitatively and suggested that radial diffusion is a major control parameter for the long-term variations of the inner portion of the outer belt. On the other hand, the simulation did not reproduce the variation of the outer portion of the outer belt, suggesting that further physical processes should be considered.