NOVELTY - The magnesium hydroxide fine particles have a BET specific surface area of 5 m2/g or more, a volume-based 50% cumulative particle diameter of 0.1-0.5 mu m as measured by laser diffraction scattering type particle size distribution measurement, a ratio of 10 or less with respect to volume-based 90% cumulative particle diameter and volume-based 10% cumulative particle diameter, and a purity of 99.5 %mass or more.    USE - Magnesium hydroxide fine particles are used for manufacturing magnesium oxide microparticles (all claimed). Uses include but are not limited to additives, resin fillers, highly functional materials, refractory components, magnetic steel sheet materials, catalysts used for wastewater treatment and exhaust-gas treatment, ceramics used for fuel cells, raw materials of fluorescent substances, raw materials of superconducting thin film substrates, tunnel magnetoresistive elements, heat-resistant layers of separators used in secondary batteries, flame retardants, modifiers of film sheets, ink fixing agents of inkjet printing paper, and insulation film materials of compacted-powder iron cores.    ADVANTAGE - The magnesium hydroxide fine particles are uniform, have a small particle size and sharp particle size distribution, and can be manufactured easily and conveniently.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) magnesium oxide microparticles having a BET specific surface area of 5 m2/g or more, a volume-based 50% cumulative particle diameter of 0.1-0.5 mu m, a ratio of 10 or less with respect to volume-based 90% cumulative particle diameter and volume-based 10% cumulative particle diameter, and a purity of 99.5 %mass or more;    (2) manufacture of magnesium hydroxide fine particles, which involves preparing magnesium chloride aqueous solution, reacting the aqueous solution with 1-18N aqueous alkali solution at a reaction rate of 101-210 mol% to obtain a magnesium hydroxide slurry, maintaining a temperature of 101-200 degrees C, hydrothermally treating the slurry while stirring, filtering, washing with water, and drying; and    (3) manufacture of magnesium oxide microparticles, which involves baking the magnesium hydroxide fine particles at 500-1500 degrees C in atmospheric condition.