1. A measuring method in which a probe is brought into contact with a work to measure a size of the work, comprising: storing correction computation equations capable of correcting a geometrical measurement error inherent in a measuring apparatus, the correction computation equations being used for computation based on design values; measuring a work and computing a measured value by using the correction computation equations; optimizing as variables, by a numerical solution, a plurality of design values in the design values substituted in the correction computation equations so that an error between the computed measured value and a true value is minimized; storing the optimized correction computation equations in which optimized variables are substituted; and computing the measured value by using the optimized correction computation equations in measurement subsequently performed.