Focus detecting methods and focus detecting mechanisms of an image measuring device able to execute focus detection at high speed with high accuracy, and image measuring device having the focus detecting mechanisms are provided. A rotation driving device rotates and operates a grating filter for projecting a grating pattern to a measured object. A distance driving device adjusts the interval distance between an image pickup device and the measured object. The image of the measured object projecting the grating pattern thereto every predetermined rotating angle is picked up by the image pickup device via multiple exposure by operating the rotation driving device and the distance driving device in association with each other. A focus position is determined 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 determined on the basis of frequency area information.