Atomic hydrogen cleaning has been developed to reduce the amount of surface oxide on Ru-capped Mo/Si multilayer mirrors for EUVL. Atomic hydrogen generated by a heated W wire catalyzer was supplied to a Ru cap layer that had been lightly oxidized by ECR O2 plasma or EUV irradiation. The effectiveness of atomic hydrogen in deoxidizing it was examined by ex situ AES, XPS, and EUV absolute reflectivity measurements; and it was found that the amount of surface oxide was reduced to the initial level and that the EUV reflectivity of a multilayer degraded by oxidation recovered. In addition, the transport of atomic hydrogen thorough a winding quartz tube was demonstrated to be a promising technique. The actual density of hydrogen radicals was directly measured under various conditions so that the conditions for generating atomic hydrogen could be optimized and the required treatment time shortened.