We have studied the atomic structure and phase transition of a monolayer phase of In on Si(111), known as the hexagonal (√7×√3) phase. Low-energy electron diffraction and scanning tunneling microscopy observations revealed that the monolayer phase actually has an incommensurate structure with the In overlayer uniaxially contracted by 2% from (√7×√3). We observed the phase transition to the (√7×√7) phase at 250–210 K upon cooling. Angle-resolved photoelectron spectroscopy and in situ four-point-probe conductivity measurements demonstrated that the transition induces disappearance of the metallic surface states and a sharp drop in conductivity, respectively. These results indicate an electronic metal-insulator transition.