We developed a coherent x-ray diffraction microscopy (CXDM) system that enables us to precisely evaluate the electron density of an isolated sample. This system enables us to determine the dose per surface unit of x rays illuminated onto an isolated sample by combining incident x-ray intensity monitoring and the CXDM of a reference sample. By using this system, we determined the dose of x rays illuminated onto a nanostructured island fabricated by focused-ion-beam chemical vapor deposition and derived the electron density distribution of such a nanostructured island. A projection image of the nanostructured island with a spatial resolution of 24.1 nm and a contrast resolution higher than 2.3×10⁷ electrons / pixel was successfully reconstructed.