PURPOSE: To quickly and accurately debug an object program to be a target by restoring the register of an EML-MPU and the contents of an EML-RAM by using contents saved to a register in an ICE-RAM and a RAM saving area in the case of executing operation for going back up to a past step.

CONSTITUTION: The register of the EML-MPU 170 and the contents of the EML-RAM 130 are restored to a state held before the start of single step operation by using the contents saved in a register having area 231 of the ICE-RAM 230 and a step (n-1) or 1 in the RAM saving area 232 so as to easily go back up to a past step. For instance, the rewriting of the contents of the EML-RAM area 135 of the RAM 130 is detected, a value held immediately before the execution of a write cycle is read out and saved in the saving area 232 and a method for using a TGT-RAM 330 instead of the RAM 130 is used as a target debugging method.