A method for producing porous silica-based particles having an average diameter of 1 to 15 μm and a compacted bulk density of 0.20 to 0.76 g/cm 3, comprising the steps of: (a) preparing a two-layer separated liquid consisting of a layer of an organic silicon compound expressed by the following general formula and a layer of water, adding an organic solvent, an alkali and a surfactant into the water layer while agitating at least the water layer so that the organic silicon compound layer and the water layer are not completely mixed with each other, thereby forming a mixed aqueous solution, and hydrolyzing or partially hydrolyzing the organic silicon compound in the mixed aqueous solution to prepare silica-based particle precursors; R 1 n Si(OR 2) 4-n wherein: R 1 is a monad selected from a C 1-10 alkyl group, a C 6-10 aryl group and a C 2-10 unsaturated aliphatic group, each of which may have a substituent group, R 2 is a monad selected from a C 1-5 alkyl group and a C 2-5 acyl group, and n is an integer from 1 to 3; (b) adding sodium aluminate into the mixed aqueous solution containing the silica-based particle precursors so as to produce silica-based particles having pores, cavities or voids inside the particles; and carrying out the following step (c) or carrying out both of the following steps (d) and (e): (c) washing and drying the silica-based particles; (d) adding the silica-based particles into an alkaline aqueous solution to dissolve a part of a covering layer formed on an external peripheral surface of the particles; and (e) washing and drying the silica-based particles having a part of a covering layer dissolved; wherein step (d) is optionally preceded by the step of washing and drying the silica-based particles; wherein, in step (a), the quantity of water in the two-layer separated liquid is 3 to 50 times the quantity of the organic silicon compound on a weight basis, the quantity of the organic solvent is 0.001 to 5% by weight based on the quantity of water employed in step (a), the quantity of the alkali is 0.0001 to 0.1% by weight based on the quantity of water employed in step (a), the quantity of the surfactant is 0.005 to 1% by weight based on the quantity of water employed in step (a), and the organic silicon compound layer, the water layer, the organic solvent, the alkali and the surfactant are at a temperature in the range of 0.1 to 15°C; and wherein, in step (b), the addition of sodium aluminate is performed, whilst agitating the mixed aqueous solution, for 0.01 to 1 hour at a temperature in the range of 5 to 45°C, and the mixed aqueous solution is left, whilst being agitated, for 0.5 to 50 hours at a temperature in the range of 5 to 50°C after the sodium aluminate has been added. (a) preparing a two-layer separated liquid consisting of a layer of an organic silicon compound expressed by the following general formula and a layer of water, adding an organic solvent, an alkali and a surfactant into the water layer while agitating at least the water layer so that the organic silicon compound layer and the water layer are not completely mixed with each other, thereby forming a mixed aqueous solution, and hydrolyzing or partially hydrolyzing the organic silicon compound in the mixed aqueous solution to prepare silica-based particle precursors; R 1 n Si(OR 2) 4-n wherein: R 1 is a monad selected from a C 1-10 alkyl group, a C 6-10 aryl group and a C 2-10 unsaturated aliphatic group, each of which may have a substituent group, R 2 is a monad selected from a C 1-5 alkyl group and a C 2-5 acyl group, and n is an integer from 1 to 3; R 1 is a monad selected from a C 1-10 alkyl group, a C 6-10 aryl group and a C 2-10 unsaturated aliphatic group, each of which may have a substituent group, R 2 is a monad selected from a C 1-5 alkyl group and a C 2-5 acyl group, and n is an integer from 1 to 3; (b) adding sodium aluminate into the mixed aqueous solution containing the silica-based particle precursors so as to produce silica-based particles having pores, cavities or voids inside the particles; and (c) washing and drying the silica-based particles; (d) adding the silica-based particles into an alkaline aqueous solution to dissolve a part of a covering layer formed on an external peripheral surface of the particles; and (e) washing and drying the silica-based particles having a part of a covering layer dissolved;