We have been developing the methodology for high-resolution crystal and magnetic structure analysis using X–ray, Synchrotron radiation and Neutron beam under extreme conditions such as low temperature, high magnetic field, and high pressure. We have also studied structural phase transitions to understand the microscopic origins of functional properties in solid–state materials based on the accurate distribution analyses of the electron as well as nuclear densities. Figure shows the complex magnetic structure of magnetic ferroelectrics derived by neutron magnetic structure analysis. Cycloidal spin structure in this material is thought to be the origin of electric polarization. We also engage the development of novel neutron 2-dimensional detector for structure analysis JRR-3 reactor.