There is provided an agent for improving the growth of seedlings, having effects of raising healthy rice seedlings, such as increase of the tillering number, increase of root weight, enhancement of greenness, and increase of stem thickness.An agent for improving the growth of rice seedlings containing, as active ingredients, 5-aminolevulinic acid represented by formula (1), a derivative thereof or a salt of the acid or the derivative, and a gibberellin biosynthesis inhibitor: R2R1NCH2COCH2CH2COR3  (1) wherein R1 and R2 each independently represent a hydrogen atom, an alkyl group, an acyl group, an alkoxycarbonyl group, an aryl group or an aralkyl group; and R3 represents a hydroxyl group, an alkoxy group, an acyloxy group, an alkoxycarbonyloxy group, an aryloxy group, an aralkyloxy group or an amino group.