1. A performance evaluation device comprising: a workload controlling unit that sequentially generates, for each of a plurality of single workloads types, a plurality of workloads having different sizes on an evaluation target computer, and sequentially generates, for a combined workload type formed by combining at least two different single workload types, a plurality of combined workloads formed by combining different types of workloads corresponding to the combined workload type on the evaluation target computer; a data acquiring unit that acquires primary performance data on the evaluation target computer currently running each of the workloads, and combined performance data on the evaluation target computer currently running each of the combined workloads; a data analyzing unit that models the primary performance data acquired by the data acquiring unit to generate a primary performance model corresponding to each of the single workload types; and a model generating unit that generates a combined performance model corresponding to the combined workload type based on the combined performance data acquired by the data acquiring unit and the primary performance model generated by the data analyzing unit.