An endoscope washer disinfector washes and disinfects an endoscope with a mouth ring communicating with an endoscope duct. The washer disinfector comprises a washing disinfecting bath; a nozzle arranged with the bath, loaded into the mouth ring, and formed to supply fluid to the duct via the mouth ring; a tray on which the endoscope is mounted. In this washer disinfector, a bilateral positioning member positions the endoscope and the tray so that the mouth ring of the endoscope mounted on the tray is located at a given position on the tray; and a trilateral positioning member positions the washing disinfecting bath, the tray, and the endoscope so that, when the tray is loaded into the washing disinfecting bath, the mouth ring of the endoscope positioned at the given position on the tray by the bilateral positioning member is positioned at a position that is across is from the nozzle.