1. A method of calculating a processor utilization for two logical processors in a computer having a physical processor, the two logical processors running on the physical processor and a memory on which an operating system (OS) is loaded; wherein the physical processor is a simultaneous multithreading (SMT) processor and each of the logical processors has two operation modes including a single task mode in which only one of them is active and a multitask mode in which the two logical processors are active; wherein the OS has a processor utilization rate monitoring means, and the processor utilization rate monitoring means is configured to execute a method that comprises the steps of: defining one computation interval, for which the processor utilization for each of the logical processors is to be calculated, as an interval of an arbitrary length in which the logical processor being calculated is active from its start to end without transiting to an inactive state; dividing the computation interval into a single task mode execution interval and a multitask mode execution interval of the logical processor being calculated; calculating the single task mode execution interval and the multitask mode execution interval of the logical processor being calculated; adding the multitask mode execution interval multiplied by a predetermined multitask mode processor resource assignment ratio to the singe task mode execution interval; and taking a resultant added value as the processor utilization of the logical processor being calculated in the computation interval.