In estimating a curved surface model by approximating the shape of the board surface of a circuit board, auxiliary measurement spots are set other than measurement spots on the board surface, eligibility as a sampling displacement magnitude in estimating a curved surface model is determined according to a difference in a displacement magnitude from a work reference surface. When the sampling displacement magnitude is determined to be ineligible, a new measurement spot is reset. By this operation, a local increase and decrease in the displacement magnitude due to a discontinuity of the board surface exerts no influence on the estimation of the curved surface model, and the curved surface model approximated more closely to the shape of the actual board surface is estimated, leading to an improvement in the work quality with the working height adjusted to the proper height.