Provided is a tolerance improving agent that can increase or improve environmental stress tolerances (high-temperature tolerance, drought tolerance and the like) of plants. The tolerance improving agent is constituted of at least one kind of unsaturated carbonyl compound selected from unsaturated aldehyde and unsaturated ketone. Such unsaturated carbonyl compound is, for example, a compound having 4-9 carbon atoms, which does not have an unsaturated bond at the terminal, and may be at least one kind selected from α,β-alkenal, α,β-alkanone, γ,δ-alkenal, γ,δ-alkanone, δ,ε-alkenal, δ,ε-alkanone, ε,ζ-alkenal, and ε,ζ-alkanone.
