An image measuring instrument includes a camera that images a plurality of measured points based on a preset measuring procedure, and an illumination unit that irradiates the measured points. The image measuring instrument measures a dimension and a shape of a to-be-measured object while the illumination unit irradiates light having illumination light quantity corresponding to an illumination preset value with reference to the illumination preset value that is preliminarily set for every measured point. An illumination light quantity setting method includes a command signal output step that outputs a command signal for irradiating light having the illumination light quantity corresponding to the illumination preset value for every measured point, an offset value assigning step that assigns an offset value to the command signal to be sent to the illumination unit, and a setting step that sets the offset value to be variable.