PURPOSE: To decrease restoring frequency and to carry out the prolog, etc., at a high speed by restoring only the save contents of a register whose contents are changed in accordance with the state of a changed flag means in a back track state.

CONSTITUTION: The unchanged contents of a subject register are saved to a choice point area only at the first change of the contents of said register. Then the save contents of a register where the change of contents is shown by a changed flag means (at fail) are restored into the original register for a back track where the changed flag means is added to each register to show the register (saved to the choice point area) whose contents are changed. The contents of all subject registers are saved and this fact is stored in the corresponding changed flag means at change of the register contents. Then it is possible to restore only the registers whose contents are changed via the changed flag means at back track.