A method for producing an optically active 2-amino-phosphonoalkanoic acid salt, the method comprising: reacting, as a starting material, a DL-2-amino-phosphonoalkanoic acid represented by Formula or a hydrate thereof with an optically active basic compound other than an optically active lysine, to give a diastereomeric salt mixture comprising: a first salt (including a hydrate salt) between a D-2-amino-phosphonoalkanoic acid represented by Formula and the optically active basic compound; and a second salt (including a hydrate salt) between an L-2-amino-phosphonoalkanoic acid represented by Formula and the optically active basic compound; and subjecting the diastereomeric salt mixture to fractional crystallization to isolate one of the first and second diastereomeric salts, Formulae,, and expressed as follows: wherein m is 1, 3, or 5; and the asterisk marks an asymmetric atom, wherein m and the asterisk are as defined above, wherein m and the asterisk are as defined above; and wherein the optically active basic compound is a compound selected from the group consisting of cinchonine, cinchonidine, quinine, quinidine, an optically active arginine, an optically active phenylalaninamide, an optically active ornithine, an optically active tyrosine hydrazide, an optically active 1-phenylpropylamine, an optically active 2-phenylpropylamine, an optically active valinol, an optically active p-hydroxyphenylglycine hydrazide, an optically active p-hydroxyphenylglycine methyl ester, an optically active p-hydroxyphenylglycine ethyl ester, an optically active glucamine, an optically active leucinol, an optically active 1-phenylethylamine, and an optically active 2-amino-1-butanol. reacting, as a starting material, a DL-2-amino-phosphonoalkanoic acid represented by Formula or a hydrate thereof with an optically active basic compound other than an optically active lysine, to give a diastereomeric salt mixture comprising: a first salt (including a hydrate salt) between a D-2-amino-phosphonoalkanoic acid represented by Formula and the optically active basic compound; and a second salt (including a hydrate salt) between an L-2-amino-phosphonoalkanoic acid represented by Formula and the optically active basic compound; and a first salt (including a hydrate salt) between a D-2-amino-phosphonoalkanoic acid represented by Formula and the optically active basic compound; and a second salt (including a hydrate salt) between an L-2-amino-phosphonoalkanoic acid represented by Formula and the optically active basic compound; and subjecting the diastereomeric salt mixture to fractional crystallization to isolate one of the first and second diastereomeric salts, Formulae,, and expressed as follows: wherein m is 1, 3, or 5; and the asterisk marks an asymmetric atom, wherein m and the asterisk are as defined above, wherein m and the asterisk are as defined above; and wherein the optically active basic compound is a compound selected from the group consisting of cinchonine, cinchonidine, quinine, quinidine, an optically active arginine, an optically active phenylalaninamide, an optically active ornithine, an optically active tyrosine hydrazide, an optically active 1-phenylpropylamine, an optically active 2-phenylpropylamine, an optically active valinol, an optically active p-hydroxyphenylglycine hydrazide, an optically active p-hydroxyphenylglycine methyl ester, an optically active p-hydroxyphenylglycine ethyl ester, an optically active glucamine, an optically active leucinol, an optically active 1-phenylethylamine, and an optically active 2-amino-1-butanol.