The present invention provides: a plant growth promoter containing a phosphorylated oligosaccharide polyvalent metal salt as an active ingredient; and a plant cultivation method comprising applying a phosphorylated oligosaccharide polyvalent metal salt to plants, preferably spraying the phosphorylated oligosaccharide polyvalent metal salt on plant foliage. The plant growth promoter and the plant cultivation method according to the present invention are highly safe and can alleviate and/or prevent blossom-end rot in plants.