A method for cleaning collector mirrors in an EUV light generator in which a target is made into a plasma state and EUV light generated is collected by a collector mirror, the method being adopted to the EUV light generator for cleaning contaminants adhering thereto, the method comprising: preparing at least two collector mirrors; locating one of the mirrors at an EUV light condensing position while locating the other mirror at a cleaning position; determining whether the mirror at the cleaning position is cleaned while determining whether the mirror at the light condensing position requires cleaning; and once determined that the mirror at the cleaning position is cleaned and the mirror at the light condensing position requires cleaning, conveying the mirror at the light condensing position and requiring cleaning to the cleaning position while conveying the mirror at the cleaning position and having been cleaned to the light condensing position.