Transmutation of minor actinides and long-lived fission products using a proton accelerator has many advantages over a transmutor operated in a critical condition. The energy required for this transmutation can be reduced by multiplying the spallation neutrons in a subcritical assembly surrounding the spallation target. Study was done on the relation between the energy requirements and the multiplication factor k of the subcritical assembly, while varying the range of several parameters in the spallation target. A slightly subcritical reactor is superior to a reactor with large subcriticality in the context of the energy requirement of a small proton accelerator, the extent of radiation damage, and other safety problems. To transmute the longlived fission products without consuming much fissile material, the transmutor reactor must have a good neutron economy, which can be obtained by using a transmutor operated by a proton accelerator. Consideration is given to the use of minor actinides to improve neutronic characteristics, such as achieving a long fuel burnup rather than simply transmuting this valuable material.