PURPOSE: To reduce the number of storage areas, by detecting an infinite loop which can be caused at replacement processing of variables through the use of less storage area.

CONSTITUTION: Replaced variables NWVR are stored in a memory VRMR, the content of the variable memory VRMR is replaced with a variable NWVR replaced at that time for every variable impulse, and the variables on the memory VRMR are compared with the variables replaced. When an infinite loop takes place at the replacement processing of variables, since the replacement is repeated with a given pattern thereafter, after the interval is greater than the loop period, the data equal to the varibles of the variable memory VRMR take place in the replaced variables NWVR without fail and the infinite loop produced in the porcess of replacement process of variables can be detected.