A plant growth promoting agent which contains cellulose nanofibers and which is directly or indirectly applied to, sprinkled on, or sprayed on a plant. By using cellulose nanofibers having a specific average fiber diameter and a specific average fiber length, agglomeration of the cellulose fibers is thereby suppressed. This enables efficient insecticidal effect to be realized, wherein the insecticidally active ingredient is a non-chemosynthetic material.
