Alkali agent-containing particles comprising (a) particles containing an alkali agent and having an average particle size of from 150 to 1000 μm, and a layer of (b) clay mineral particles exhibiting a water impregnation capacity of 200% by mass or more as calculated according to the following method and having an average particle size of from 10 to 50 μm, the layer being formed on a surface of the respective particles (a): Method for calculation of water impregnation capacity: A dispersion of the clay mineral particles (b) obtained after measuring a swelling power of the particles (b) according to a Standard Testing Method of Japan Bentonite Manufactures Association "Swelling Test Method for Bentonite (Powder)" (JBAS-104-77) (volumetric method) is subjected to suction filtration (under 0.5 MPa for 2 h; filter paper: "Type No. 4A" having a diameter of 90 mm available from Advantec Toyo Kaisha, Ltd.) using a Buchner funnel to measure a mass of a residue of the clay mineral particles (b) on the filter paper; the clay mineral particles (b) are dried according to a drying method of a sample prescribed in JIS K0068 "Method for Measuring Water Content of Chemical Products; 7: Loss-on-Drying Method" to measure a mass of the dried clay mineral particles (b); and a water impregnation capacity of the clay mineral particles (b) is calculated from the thus measured mass and mass according to the following formula: Water impregnation capacity % = A ⋅ B / B × 100. Method for calculation of water impregnation capacity: A dispersion of the clay mineral particles (b) obtained after measuring a swelling power of the particles (b) according to a Standard Testing Method of Japan Bentonite Manufactures Association "Swelling Test Method for Bentonite (Powder)" (JBAS-104-77) (volumetric method) is subjected to suction filtration (under 0.5 MPa for 2 h; filter paper: "Type No. 4A" having a diameter of 90 mm available from Advantec Toyo Kaisha, Ltd.) using a Buchner funnel to measure a mass of a residue of the clay mineral particles (b) on the filter paper; the clay mineral particles (b) are dried according to a drying method of a sample prescribed in JIS K0068 "Method for Measuring Water Content of Chemical Products; 7: Loss-on-Drying Method" to measure a mass of the dried clay mineral particles (b); and a water impregnation capacity of the clay mineral particles (b) is calculated from the thus measured mass and mass according to the following formula: Water impregnation capacity % = A ⋅ B / B × 100. A dispersion of the clay mineral particles (b) obtained after measuring a swelling power of the particles (b) according to a Standard Testing Method of Japan Bentonite Manufactures Association "Swelling Test Method for Bentonite (Powder)" (JBAS-104-77) (volumetric method) is subjected to suction filtration (under 0.5 MPa for 2 h; filter paper: "Type No. 4A" having a diameter of 90 mm available from Advantec Toyo Kaisha, Ltd.) using a Buchner funnel to measure a mass of a residue of the clay mineral particles (b) on the filter paper; the clay mineral particles (b) are dried according to a drying method of a sample prescribed in JIS K0068 "Method for Measuring Water Content of Chemical Products; 7: Loss-on-Drying Method" to measure a mass of the dried clay mineral particles (b); and a water impregnation capacity of the clay mineral particles (b) is calculated from the thus measured mass and mass according to the following formula: Water impregnation capacity % = A ⋅ B / B × 100.