1. A virtual machine system, comprising: a memory; and a processor configured to execute a first stage VM (Virtual Machine) and a second stage VM generated on the first stage VM, the processor being coupled to the memory, wherein the processor is configured to perform a first determination as to whether to physically instruct to start execution caused by a virtual execution start of the second stage VM and a second determination as to whether a physical end is detected as a result of a virtual end of the second stage VM, and calculate an execution time of the second stage VM based on results of the first determination and the second determination.