PURPOSE: To prevent the trouble of the wave-form formition of a worked surface by applying the fuzzy estimation, having at least one of the copying information obtained during the copying operation as fuzzy variable and obtaining the optimum value of the parameters for controlling the normal line speed or tangential line speed.

CONSTITUTION: A fuzzy estimation part 11 carries out fuzzy estimation, receiving the side surface degree signal Sd and the displacement error Δ as input, and the attenuation quantity is estimated from the membership function of the gain attenuation degree VNG of the normal line speed. An interpretation part 12 defuzzy processes the estimation value supplied from the obtained membership function, and outputs the gain attenuation degree VNG of the actual normal line speed. If the elevation angle α is 75°, and the error displacement quantity Δ is 30μm, the pertinent rule is the rules b, c, d, e, and f. When the above-described input condition is applied to the membership function, the membership function of the gain attenuation degree VNG is obtained. When this membership function is defuzzy-processed by the center of gravity method, etc., the actual gain attenuation degree VNG can be obtained.