1. A virtual machine system including: a processor having a first mode and a second mode; a first operating system executed by the processor in the first mode; and a second operating system executed by the processor in the second mode, the virtual machine system comprising: a write control unit configured to permit writing of data into a predetermined secure storage area in an external main memory, only when the processor is in the first mode; and a cache memory having a plurality of ways for storing data read by the processor from the main memory, the cache memory including: a data storage unit configured to, when the processor has read data from the main memory, store the data into any of the plurality of ways that is ready to newly store data, in a manner that allows for identification of whether the data has been read from the secure storage area; and a write-back unit configured to identify whether data has been read from the secure storage area, and write back data stored in at least one of the ways to the main memory with use of a predetermined algorithm according to a result of the identification, the writing back being performed in a manner that reduces the number of times data stored in each of the ways is intermittently written back to the secure storage area, the writing back to the secure storage area occurring when the processor executing the second operating system accesses the main memory, and being for causing the at least one of the ways to be ready to newly store data.