1. An image measurement device comprising: a stage for placing a workpiece; an illumination device that illuminates the workpiece on the stage; a camera that photographs the workpiece on the stage to generate a workpiece image; a measurement target place specifying unit that specifies a measurement target place on the workpiece image; an illumination condition storage unit that holds two or more illumination conditions; an imaging control unit that controls the camera and the illumination device to acquire two or more of the workpiece images sequentially photographed while making the illumination condition different; a workpiece image display unit that displays the acquired plurality of workpiece images; a workpiece image selecting unit that selects any one of the displayed workpiece images; an illumination condition deciding unit that decides the illumination condition based on the selected workpiece image; a measurement setting storage unit that holds, as measurement setting information, two or more measurement target places specified on the same workpiece image by the measurement target place specifying unit and an illumination condition decided by the illumination condition deciding unit with respect to each of these measurement target places, wherein the illumination condition is held in association with the measurement target place in the measurement setting storage unit; and a dimension calculating unit that extracts an edge of the measurement target place based on the workpiece image photographed on the decided illumination condition, and obtains a dimension of the measurement target place based on the extracted edge; wherein the imaging control unit repeatedly acquires a workpiece image photographed on the illumination condition in association with each of the measurement target places held as the measurement setting information, and the dimension calculating unit performs edge extraction with respect to each of the measurement target places held as the measurement setting information on the plurality of workpiece images acquired by the imaging control unit and obtains a dimension between edges respectively extracted from two or more workpiece images with mutually different illumination conditions and positions of the measurement target place.