A wrench includes a handle and a set of driving members to be selected to be coupled to the handle. An image generator calculates a deviation amount indicating the position deviation between images represented by the image data acquired by the capturing units, and generates image data of a multi-channel image in which the position deviation between the images represented by the image data acquired by the capturing units has been corrected using the deviation amount. The instruments are washed by spraying them with the washing liquid. Typically, a cleaning cycle involves the use of an acid, such as phosphoric acid, to facilitate chemical removal of scale which is mineral deposits resulting from the use of hard water in the boiler.