Abstract: A magnesium hydroxide satisfies: (A) primary particles with an average width as measured using a SEM method of between 0.1 ?m and 0.7 ?m inclusive; (B) a degree of monodispersity of 50% or greater wherein degree of monodispersity (%)=(average width of primary particles as measured using the SEM method/average width of secondary particles as measured using a laser diffraction method)×100; (C) a ratio D90/D10 of the volume-based cumulative 90% particle diameter (D90) to the volume-based cumulative 10% particle diameter (D10) as measured using a laser diffraction method of 10 or less; and (D) a lattice strain in the <101> direction as measured using an X-ray diffraction method of 3×10-3 or less. A nonaqueous secondary battery separator using the magnesium hydroxide and a nonaqueous secondary battery using the separator are provided. Improved heat resistance and smoking suppressibility of a nonaqueous secondary battery are disclosed.