The invention provides a boot method capable of reducing boot time even in the case of a change in the configuration of boot files. A boot file is booted from a hard disk drive in a computer equipped with a multitasking operating system. A plurality of tasks are observed, the tasks being created and sequentially executed in order to read out boot files. This observation is carried out at the corresponding boot and a log in the past boot is not used. On the basis of the observed boot files, a prefetch boot file is selected. The entire selected prefetch boot file is filled into a boot cache. The boot file is loaded from the boot cache to a main memory.