Abstract: In a debugging system for use in realizing simultaneously complete observation of internal operation and reproduction of malfunction actually caused in a target system which includes an integrated circuit, such as a microprocessor, contents of a memory and an internal initial state of the target system are snooped and stored in a snoop unit and a trace memory, respectively, both of which are included in a probe unit attached to the target system. The internal operation and the reproduction are simulated by a software simulator model by the use of the contents and the internal initial state which are stored in the trace memory and which are sent from the probe unit.