PURPOSE: To eliminate sudden change due to noise by processing three sampling values sampled continuously and sequentially at a prescribed time interval as specified.

CONSTITUTION: A processor CPU stores a sampling value d3 obtained from an A/D converter to a variable memory RAM. Then the operation of mean value da=(d1+d3)/2 is executed by using a sampling value d1 of 2nd preceding time. Then the discrimination of dad2? is conducted with the preceding sampling d2 stored in the RAM. When the result is NO, the discrimination of d1d2? and d3d2? is executed. When all the results of discrimination are NO, that is, dissident, the d2 is replaced into a mean value da, which is transmitted to a D/A converter. Through the processing above, sudden change due to noise is eliminated.