To provide a focus detecting method of an image measuring device able to execute the focus detection at high speed with high accuracy, and to provide a focus detecting mechanism and the image measuring device having the focus detection mechanism.

A rotation driving means 40 rotates and operates a grating filter 26 for projecting a grating pattern to a measured object. A distance driving means 50 adjusts the interval distance between an image pickup means 30 and the measured object (W). The image of the measured object (W) projecting the grating pattern thereto every predetermined rotating angle is picked up by the image pickup means (30) by means of multiple exposure by operating the rotation driving means 40 and the distance driving means 50 in association with each other. A focus position is calculated from a contrast value of the grating pattern within the picked-up image information. Accordingly, it is not necessary to pick-up plural images in plural distance positions and process the plural images. Therefore, times taken in the image picking-up and the image processing can be shortened and a focus detecting speed can be increased. Further, the focus detection can be performed with high accuracy since the contrast value is calculated on the basis of frequency area information.