An image converter receives image data of an object and performs a kind of image conversion on the image data to generate converted image data for display. A measurement section measures at least one spatial characteristic of the object based on: the converted image data, and optical data, which relates to optical characteristics of an optical system through which the image data of the object has been obtained, and which is made to correspond to the kind of image conversion performed by the image converter. Alternatively, a measurement section measures at least one spatial characteristic of the object by relating, based on the kind of image conversion, coordinates of the converted image data to coordinates of the image data of the object before the image conversion, and by measuring the at least one spatial characteristic based on the coordinates of the image data of the object before the image conversion.