One of the most unique architectural particularities in the Khmer monument is the gallery, appearing from about 11 century in the Khleang style of the Khmer architectural history. The architectonic evolution regarding the corbel arch, window, and connective beam in a gallery indicates the identity of the Angkor monuments, nevertheless earlier studies have lacked such architectonic viewpoints. In this paper each phase of the architectonic evolution and its spatial meaning is looked at with several monuments in the Angkor archeological site. In conclusion, this paper indicates the followings. Firstly the appearance of a gallery is not derived from the long-buildings around the sanctuary but from the new ideas giving symbolism of Meru and keeping the spatial holiness of the site. Secondly a closed space with a gallery was changed into an open space in mountain-monument type in later times, and it proves the discovery of the new conception by Khmer architects in terms of landscape. Finally the architectonic concepts of a gallery derive mostly from wooden architectonics. When faced with technical problems in masonry work, it is likely that Khmer architects overcame the architectonic difficulties in view of the wooden architectural concept and techniques.