A horizontally moving automatic outer surface cleaning apparatus comprising horizontal rails provided above and below a surface to be cleaned such as a window glass and a carrier having engaging means for engaging with the horizontal rails for enabling the carrier to be guided along the rails. On the carrier, there are mounted drive means for driving the carrier to move along the horizontal rails, water spray means for spraying water over the surface to be cleaned, wiper means for cleaning the surface by wiping the surface and a soiled water tank containing soiled water collected as a result of wiping by the wiper means. In a preferred embodiment, the carrier consists of a main carrier and a work carrier coupled to the main carrier in such a manner that the carrier can move along a curved portion which can be formed in the horizontal rails.