We have performed an angle-resolved photoemission study of the two-leg ladder system Sr14−xCaxCu24O41 with x= 0 and 11. "Underlying Fermi surfaces" determined from low energy spectral weight mapping indicates the quasi-one dimensional nature of the electronic structure. Energy gap caused by the charge density wave has been observed for x=0 and the gap tends to close with Ca substitution. The absence of a quasi-particle peak even in x=11 is in contrast to the twodimensional high-Tc cuprates, implying strong carrier localization related to the hole crystalization.PACS numbers: 74.25.Jb, 71.18.+y, 74.72.Dn, Since the discovery of the high-T c superconductor in layered cuprates, one-dimensional (1D) cuprates have also attracted much interest 1 . Particularly, Cu-O ladders with an even number of coupled Cu-O chains are predicted to have a finite spin gap reminiscent of that in the underdoped high-T c cuprates, and have the possibility of superconductivity with hole doping 2 . Ladder compounds such as Sr 14−x Ca x Cu 24 O 41 (Sr14-24-41) and LaCuO 2.5 in which two-dimensional CuO 2 planes is reorganized into 1D segments have long been envisioned to bridge 1D chain systems, where Luttinger liquid behaviors are predicted, and 2D plane systems, which is the stage of the high-T c superconductivity. Sr14-24-41 is composed of alternating stacks of the plane of edge sharing CuO 2 chains, (Sr, Ca) layer, and the plane including two-leg Cu 2 O 3 ladders. With Ca substitution, holes are transferred from the chain sites to the ladder sites 3 . From the optical reflectivity measurement of Sr14-24-41 at room temperature, the number of holes doped into the Cu 2 O 3 ladder plane is already 0.07/Cu atom for x=0, which should be enough to realize superconductivity (SC) in 2D cuprate 3 . However, the x=0 samples are insulating and show an activated behavior in the resistivity 4 . Upon Ca substitution for Sr, holes localized in the chains are transferred to the ladders and induce mobile carriers on the ladder. When x > 11, the mobile carriers exhibit superconductivity under high pressure 5 . To understand the electronic structure of such quasi-1D systems, angle-resolved photoemission spectroscopy (ARPES) is a powerful tool because it provides us with momentum-resolved information about the electronic states. In the previous ARPES experiments on the ladder system Sr14-24-41, two dispersive features along the ladder and chain directions were observed and assigned to the ladder-derived band (near E F ) and the chain-derived band (∼ 1 eV below E F ), respectively 6,7 . However, in the previous results, only one-dimensional momentum dependence along the ladder/chain direction has been studied in momentum space. Since superconductivity in this system appears when application of pressure causes a dimensional crossover from one to two as reflected in electrical resistivity 8 , two-dimensional electronic structure has been thought to be crucial to the superconductivity. In the present work, we have performed ARPES experiments on the two-leg ladd...