1. A grinding machine for grinding a workpiece, which has been set on a chuck top surface, by moving a rotating grinding wheel in relation to the workpiece, the grinding machine comprising: a microscope configured to be vertically moveable; a charge-coupled device (CCD) camera configured to take an image viewed through the microscope; and an image processor configured to process the image taken by the CCD camera to measure a vertical distance between a reference plane of the microscope and an object of the microscope; wherein the image processor is adapted to measure the vertical distance between the reference plane of the microscope and the object of the microscope based on sharpness of the image, which corresponds to how clear the microscope is focused, the image processor is connected to a numerical control (NC) device, the image processor is adapted to receive, from the NC device, coordinates of the reference plane of the microscope positioned at a point at which the vertical distance is being measured, to determine coordinates of the object of the microscope based on the coordinates of the reference plane of the microscope and the vertical distance that has been measured, and to send the coordinates of the object of the microscope to the NC device, the NC device is connected to a drive controller for controlling vertical movement of the microscope and is adapted to control the drive controller, the NC device causes the microscope to move vertically at least once via the drive controller, the image processor is adapted to continuously receive at predetermined time intervals a plurality of images viewed through the vertically moving microscope and to specify an image having a highest degree of sharpness based on the sharpness of each image it received, thereby extracting a vertical region including a vertical position corresponding to the image with the highest degree of sharpness, the NC device next causes the microscope to move vertically at least once in the extracted vertical region via the drive controller, and the image processor is adapted to continuously receive at predetermined time intervals a plurality of images viewed through the microscope moving vertically in the extracted vertical region and to specify an image having a highest degree of sharpness based on the sharpness of each image it received, thereby measuring the vertical distance between the reference plane of the microscope and the object of the microscope.