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; 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.