We have executed scanning tunneling microscopy (STM) observation of triple-decker Y2Pc3 molecules deposited on a Au(111) surface. It is shown that the triple-decker phthalocyanine molecule, tris(phthalocyaninato)yttrium, can be successfully transferred to the Au(111) surface by a sublimation method under an ultra high vacuum condition. A monolayer film of Y2Pc3 was observed with a height of ∼0.55nm from the bare Au(111) surface. The molecules were adsorbed with a flat-lying configuration and a pseudosquare lattice was formed. We propose a temporal model of the lattice with two unit vectors a and b, where a is parallel to the [11̄0] direction with a length of 5a [a is the nearest-neighbor distance of the Au(111)] and b is parallel to the [112̄] direction with a length of 3√3a. Inside each molecule, eight-lobe bright protrusions were observed in occupied-state images, which correspond to the high-density-of- states (DOS) area on both sides of the four phenyl rings in the top phthalocyanine (Pc). Scanning tunneling spectroscopy (STS) observation showed that the HOMO and LUMO levels are at the ligand position and the metalcenter position, indicating a small contribution of the metal Y to the DOS of the top layer.