To safely and efficiently clean the outdoor surface of a windowpane from indoors.

A sealed space 14 is formed over the indoor surface of a windowpane 12 and cold wind of a temperature such that the temperature of the outdoor surface of the windowpane 12 drops to or below dew point is fed into the sealed space 14 to form water droplets 20 on the outdoor surface of the windowpane 12 and to make them cling thereto. The windowpane 12 and the water droplets 20 clinging thereto are ultrasonically vibrated by mounting an ultrasonic vibrator 24 on the indoor surface of the windowpane 12. Thus, as dust clinging to the outdoor surface of the windowpane 12 is washed with the water droplets, the dust is removed together with the water droplets.