1. A debugging kernel system comprising: a host system comprising a program debugging tool; and a target machine having a kernel, said target machine comprising: a product subsystem; and a target subsystem having a debugging kernel, said debugging kernel comprising a thread scheduling function and an interrupt process function on said target subsystem: wherein said debugging kernel controls said product subsystem and said target subsystem which has a function operating thereon that is loaded as a thread for control thereby, and said thread scheduling function assigns a priority to the threads and determines a process to be executed next by scheduling according to the priority, and said interrupt process function executes an interrupt process by performing a predetermined process between said product subsystem and said target subsystem so as to prevent said product subsystem from preempting a process of said debugging kernel.