When measuring the dimensions of a tool having a cutting portion at an edge of a planar portion, first a plurality of points are measured on the planar portion using a measuring probe, then a planar formula for a measurement plane containing the planar portion is determined from these measured values. Next, the measuring probe is moved along the measurement plane expressed by the planar formula from the exterior of the cutting portion toward the cutting portion until the measuring probe contacts the cutting portion. The position of the cutting portion is determined based on the position of the measuring probe when the measuring probe comes into contact with the cutting portion.