Use of an agricultural agent for providing induced systemic resistance against plant diseases, which comprises a 2,5-diketopiperazine compound selected from the following or an agriculturally acceptable salt thereof as an active ingredient: 3-hydroxymethyl-6-(2-methylpropyl)-2,5-diketopiperazine, 3-(2-methylpropyl)-2,5-diketopiperazine, 3-(1-hydroxyethyl)-6-(2-methylpropyl)-2,5-diketopiperazine, 3-isopropyl-2,5-diketopiperazine, 3-(indol-3-ylmethyl)-2,5-diketopiperazine, 2,5-diketopiperazine, 3-methyl-2,5-diketopiperazine, 3,6-dimethyl-2,5-diketopiperazine, 3-methyl-6-(2-methylethyl)-2,5-diketopiperazine, 3-methyl-6-(4-hydroxybenzyl)-2,5-diketopiperazine, 3-(1-hydroxyethyl)-6-(2-mercaptomethyl)-2,5-diketopiperazine, 3-(1-hydroxyethyl)-6-(2-methylthiomethyl)-2,5-diketopiperazine, 3-(1-hydroxyethyl)-6-(2-tritylthiomethyl)-2,5-diketopiperazine, 3-(1-hydroxyethyl)-6-benzyl-2,5-diketopiperazine, 3-(7-hydroxyethyl)-6-(4-hydroxybenzyl)-2,5-diketopiperazine, 3-(1-hydroxyethyl)-6-(indol-3-ylmethyl)-2,5-diketopiperazine, 3-(2-methylpropyl)-6-(2-mercaptornethyl)-2,5-diketopiperazine, 3-(2-methylpropyl)-6-(2-methylthiomethyl)-2,5-diketopiperazine, 3-(2-methylpropyl)-6-(2-tritylthiamethyl)-2,5-diketopiperazine, 3-(2-methylpropyl)-6-benzyl-2,5-diketopiperazine, 3-(2-methylpropyl)-6-(4-hydroxybenzyl)-2,5-diketopiperazine, 3-(2-methylpropyl)-6-(indol-3-ylmethyl)-2,5-diketopiperazine, 3-(4-hydroxybenzyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-benzylhexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-methylhexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-isopropylhexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(2-methylpropyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(1-methylpropyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(indol-3-ylmethyl)hexahydropyrrolo[2,2-a]pyrazin-1,4-dione, octahydropyrrolo[1,2-a:1',2'-d]pyrazin-5,10-dione, 3-hydroxymethylhexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(1-hydroxyethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(2-carbamoylethyl)hexahydropyrrolo[1/2-a]pyrazin-1/4-dione, 3-(2-methylthioethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-carbamoylmethylhexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-mercaptomethylhexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-methylthiomethylhexahydropyrrolo[1,2-a] pyrazin-1,4-dione, 3-tritylthiomethylhexahydropyrrolo[1,2-a] pyrazin-1,4-dione, 3-(2-methoxycarbonylethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(2-methoxycarbonylmethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(2-aminoethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(2-carboxyethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(2-carboxymethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(imidazol-4-ylmethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione and 3-(2-acetylaminoethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione. 3-hydroxymethyl-6-(2-methylpropyl)-2,5-diketopiperazine, 3-(2-methylpropyl)-2,5-diketopiperazine, 3-(1-hydroxyethyl)-6-(2-methylpropyl)-2,5-diketopiperazine, 3-isopropyl-2,5-diketopiperazine, 3-(indol-3-ylmethyl)-2,5-diketopiperazine, 2,5-diketopiperazine, 3-methyl-2,5-diketopiperazine, 3,6-dimethyl-2,5-diketopiperazine, 3-methyl-6-(2-methylethyl)-2,5-diketopiperazine, 3-methyl-6-(4-hydroxybenzyl)-2,5-diketopiperazine, 3-(1-hydroxyethyl)-6-(2-mercaptomethyl)-2,5-diketopiperazine, 3-(1-hydroxyethyl)-6-(2-methylthiomethyl)-2,5-diketopiperazine, 3-(1-hydroxyethyl)-6-(2-tritylthiomethyl)-2,5-diketopiperazine, 3-(1-hydroxyethyl)-6-benzyl-2,5-diketopiperazine, 3-(7-hydroxyethyl)-6-(4-hydroxybenzyl)-2,5-diketopiperazine, 3-(1-hydroxyethyl)-6-(indol-3-ylmethyl)-2,5-diketopiperazine, 3-(2-methylpropyl)-6-(2-mercaptornethyl)-2,5-diketopiperazine, 3-(2-methylpropyl)-6-(2-methylthiomethyl)-2,5-diketopiperazine, 3-(2-methylpropyl)-6-(2-tritylthiamethyl)-2,5-diketopiperazine, 3-(2-methylpropyl)-6-benzyl-2,5-diketopiperazine, 3-(2-methylpropyl)-6-(4-hydroxybenzyl)-2,5-diketopiperazine, 3-(2-methylpropyl)-6-(indol-3-ylmethyl)-2,5-diketopiperazine, 3-(4-hydroxybenzyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-benzylhexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-methylhexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-isopropylhexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(2-methylpropyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(1-methylpropyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(indol-3-ylmethyl)hexahydropyrrolo[2,2-a]pyrazin-1,4-dione, octahydropyrrolo[1,2-a:1',2'-d]pyrazin-5,10-dione, 3-hydroxymethylhexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(1-hydroxyethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(2-carbamoylethyl)hexahydropyrrolo[1/2-a]pyrazin-1/4-dione, 3-(2-methylthioethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-carbamoylmethylhexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-mercaptomethylhexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-methylthiomethylhexahydropyrrolo[1,2-a] pyrazin-1,4-dione, 3-tritylthiomethylhexahydropyrrolo[1,2-a] pyrazin-1,4-dione, 3-(2-methoxycarbonylethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(2-methoxycarbonylmethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(2-aminoethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(2-carboxyethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(2-carboxymethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione, 3-(imidazol-4-ylmethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione and 3-(2-acetylaminoethyl)hexahydropyrrolo[1,2-a]pyrazin-1,4-dione.