A process for producing a β-type zeolite, the process comprising crystallizing the β-type zeolite from a starting material mixture in a closed pressure vessel at a temperature of 100°C to 180°C, the starting material mixture containing a secondary and/or a tertiary alkylamine having 5 or more carbon atoms in the presence of a silica source, an aluminum source, an alkali, a structure-directing agent, and water, with the following starting material proportions: SiO 2 /Al 2 O 3 molar ratio, 20-40, Alkali/SiO 2 molar ratio, 0-1 H 2 O/SiO 2 molar ratio, 7-15 SDA/SiO 2 molar ratio, 0.05-0.3 Secondary/tertiary alkylamine having 5 or more carbon atoms/SiO 2 molar ratio, 0.01-1.0, wherein the secondary and/or tertiary alkylamine having 5 or more carbon atoms is at least one selected from triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, N-(2-ethylamino)piperazine, methylhydroxyethylpiperazine, bis(dimethylaminoethyl) ether, pentamethyldiethylenetriamine, trimethylaminoethylpiperazine, and tetramethylethylenediamine, wherein the structure-directing agent is selected from a compound having a tetraethylammonium cation, octamethylenebisquinuclidium, α,α'-diquinudidium-p-xylene, α,α'-diquinuclidium-m-xylene, α,α'-diquinuclidium-o-xylene, 1,4-diazabicyclo-[2,2,2]octane, 1,3,3,N,N-pentamethyl-6-azonium bicyclo[3,2,1]octane or N,N-diethyl-1,3,3-trimethyl-6-azonium bicyclo[3,2,1]octane cation, wherein no fluorine-containing compound is used in the process, after completion of the crystallization, the reaction mixture being allowed to cool and subjected to solid-liquid separation, washing the solid with pure water and drying at a temperature of 100°C to 150°C, and thereafter, burning the dried solid at a temperature of 400°C to 650°C for 1 to 10 hours to remove the structure-directing agent, thereby obtaining the β-type zeolite, said β-type zeolite having an SiO2/Al2O3 molar ratio of 20 or higher but less than 30, having an average particle diameter of 0.35-0.50 μm in an examination with an electron microscope, wherein the average particle diameter is an average value of the Feret diameters of 30 randomly selected particles, having a fluorine content of 0.1 % by weight or lower, and having a crystallite diameter, determined after SDA removal by burning but before a hydrothermal durability treatment, of 20 nm or larger, the crystallite diameter being determined from the half-value width of the diffraction line profile for the plane appearing at around 2θ=22.4° in X-ray powder diffractometry using Scherrer's equation as described in the description, and wherein the β-type zeolite shows a change in crystallite diameter through the hydrothermal durability treatment of less than 10%, wherein the hydrothermal durability treatment is a 20 hour heat treatment at 700°C in a stream of air containing 10% by volume water vapor. SiO 2 /Al 2 O 3 molar ratio, 20-40, Alkali/SiO 2 molar ratio, 0-1 H 2 O/SiO 2 molar ratio, 7-15 SDA/SiO 2 molar ratio, 0.05-0.3 Secondary/tertiary alkylamine having 5 or more carbon atoms/SiO 2 molar ratio, 0.01-1.0, wherein the secondary and/or tertiary alkylamine having 5 or more carbon atoms is at least one selected from triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, N-(2-ethylamino)piperazine, methylhydroxyethylpiperazine, bis(dimethylaminoethyl) ether, pentamethyldiethylenetriamine, trimethylaminoethylpiperazine, and tetramethylethylenediamine, wherein the structure-directing agent is selected from a compound having a tetraethylammonium cation, octamethylenebisquinuclidium, α,α'-diquinudidium-p-xylene, α,α'-diquinuclidium-m-xylene, α,α'-diquinuclidium-o-xylene, 1,4-diazabicyclo-[2,2,2]octane, 1,3,3,N,N-pentamethyl-6-azonium bicyclo[3,2,1]octane or N,N-diethyl-1,3,3-trimethyl-6-azonium bicyclo[3,2,1]octane cation, wherein no fluorine-containing compound is used in the process, after completion of the crystallization, the reaction mixture being allowed to cool and subjected to solid-liquid separation, washing the solid with pure water and drying at a temperature of 100°C to 150°C, and thereafter, burning the dried solid at a temperature of 400°C to 650°C for 1 to 10 hours to remove the structure-directing agent, thereby obtaining the β-type zeolite, said β-type zeolite having an SiO2/Al2O3 molar ratio of 20 or higher but less than 30, having an average particle diameter of 0.35-0.50 μm in an examination with an electron microscope, wherein the average particle diameter is an average value of the Feret diameters of 30 randomly selected particles, having a fluorine content of 0.1 % by weight or lower, and having a crystallite diameter, determined after SDA removal by burning but before a hydrothermal durability treatment, of 20 nm or larger, the crystallite diameter being determined from the half-value width of the diffraction line profile for the plane appearing at around 2θ=22.4° in X-ray powder diffractometry using Scherrer's equation as described in the description, and wherein the β-type zeolite shows a change in crystallite diameter through the hydrothermal durability treatment of less than 10%, wherein the hydrothermal durability treatment is a 20 hour heat treatment at 700°C in a stream of air containing 10% by volume water vapor.