Plant growth accelerator comprising, as an active ingredient, 5-amino-4-hydroxypentanoic acid of formula (1) €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒH 2 NCH 2 CH (OH) CH 2 CH 2 COOR 1 €ƒ€ƒ€ƒ€ƒ€ƒ(1) wherein R 1 represents a hydrogen atom or a hydrocarbon group having 1 to 10 carbon atoms, a derivative thereof or a salt thereof.