1. A CPU emulation system, comprising: a plurality of virtual CPUs each operating on a different physical CPU; an instruction sequence selecting section for selecting an instruction sequence to be optimized; a virtual CPU selecting section for selecting one of the plurality of virtual CPUs, which is to perform optimization processing of the selected instruction sequence, based on usage rates of the plurality of virtual CPUs; and an optimization level selecting section for determining an optimization level of the optimization processing that is to be executed by the selected one of the plurality of virtual CPUs, and giving a direction to perform the optimization processing to the selected one of the plurality of virtual CPUs.