An object of the present invention is to provide a niobium monoxide able to realize large capacitance in a miniature sized capacitor. The invention relates to a niobium monoxide having a porous structure comprising particles, characterized in that the niobium monoxide has a full-width at half maximum of an X-ray diffraction peak corresponding to a (111) plane or an X-ray diffraction peak corresponding to a (200) plane of 0.21° to 1.0°. The niobium monoxide has a large specific surface area and porosity, and is especially suitable for use in a capacitor.