Electrical resistivity measurements under high pressures up to 29 GPa were performed for oxypnictide compound LaFeAsO. We found a pressure-induced superconductivity in LaFeAsO. The maximum value of T c is 21 K at $12 GPa. The pressure dependence of the T c is similar to those of LaFeAsO 1Àx F x series reported previously.KEYWORDS: superconductivity, high pressure, electrical resistivity, oxypnictide, LaFeAsO DOI: 10.1143/JPSJ.77.113712Recent discovery of Fe-based oxypnictide superconductor LaFeAsO 1Àx F x with superconducting transition temperature T c of $26 K has attracted great interests in the field of condensed matter physics.1) The crystal structure of LaFeAsO 1Àx F x is a tetragonal ZrCuSiAs-type structure (P4=nmm), and is composed of a stack of insulating LaO and conducting FeAs layers. It is thought that conductive carriers are confined two dimensionally in the FeAs layers and the carrier concentration can be increased by the substitution of O 2À by F À . The parent compound LaFeAsO is metallic and undergoes a structural phase transition at $160 K and a magnetic phase transition at $140 K.2,3) The crystal structure changes from the tetragonal to orthorhombic, then the compound exhibits a spin density wave type antiferromagnetic (AFM) ordering. The structural distortion and the magnetic ordering are suppressed by the F substitution, resulting in the emergence of the superconductivity.Right after the discovery of the superconductivity in LaFeAsO 1Àx F x , it was reported that the T c of LaFeAsO 0:89 F 0:11 shows large pressure effect and increases to a maximum of 43 K at 4 GPa, suggesting that the lattice contraction is effective in the enhancement of the T c of oxypnictide compounds. 4) Actually, RFeAsO (R = rare earth) having smaller R atom than La atom shows the T c over 50 K. 5,6) It is known that external pressure can control carrier concentration in CuO 2 plane of high-T c cupper oxides. 7) In order to understand the effect of external pressure on T c in LaFeAsO 1Àx F x and clarify similarities and differences between the external pressure effect and the F substitution effect, we have performed electrical resistivity measurements under high pressures up to 29 GPa for the parent compound LaFeAsO. In this paper, we report a pressure-induced superconductivity in LaFeAsO with a maximum T c of 21 K.A polycrystalline sample was prepared by a solid state reaction method.1,2) The sample was confirmed to be mostly composed of a single phase of the ZrCuSiAs-type tetragonal structure with lattice constants of a ¼ 0:403268ð1Þ nm and c ¼ 0:874111ð4Þ nm at room temperature.2) Electrical resistivity measurements under high pressures up to 1.5 and 12 GPa were performed by a standard dc four-probe technique using a piston-cylinder type pressure cell and a cubic anvil cell, respectively. 8) A liquid pressure-transmitting medium (Fluorinert FC70 : FC77 ¼ 1 : 1) was used to maintain hydrostatic conditions. A diamond anvil cell (DAC) made of CuBe alloy was used for electrical resistivity measurements under high pressure...