Arbitration of IO accesses and band control based on the priority of virtual servers is enabled while curbing performance overhead during IO sharing among the virtual servers. A virtual machine system includes a CPU, a memory, a hypervisor that generates plural virtual servers, and an IO controller that controls an IO interface. The IO controller includes: a DMA receiving unit that receives DMA requests from the IO interface; a decoder that decodes received DMA requests and locates the corresponding virtual servers; a DMA monitoring counter that monitors DMA processing status for each of the virtual servers; a threshold register set in advance for each of the virtual servers; and a priority deciding unit that compares the DMA monitoring counter and the value of the threshold register, and based on processing priority obtained as a result of the comparison, decides the priority of processing of the received DMA requests.