There is provided a novel alkoxymagnesium which, when used as a constituent of a solid catalyst component for olefin polymerization to polymerize an olefin, may reduce the formation rate of a fine powder and may form a polymer having an excellent particle size distribution under high polymerization activity. The alkoxymagnesium is characterized by comprising secondary particles each of which is an aggregate of primary particles having an average particle diameter of less than 1 μm and by having a ratio represented by the average particle diameter of the primary particles/the average particle diameter of the secondary particles of 0.1 or less, a total pore volume of 0.5 to 1 cm3/g, a specific surface area of less than 50 m2/g, and a particle size distribution index (SPAN) 1 or less.