Abstract: An object of the present invention is to provide an agent effective for the treatment and/or prevention of motor neuron disease such as amyotrophic lateral sclerosis (ALS). The present invention provides a therapeutic and/or preventive agent for motor neuron disease comprising the following oligopeptide shown in any of (a) to (c) or a pharmaceutically acceptable salt thereof as an active ingredient: (a) an oligopeptide consisting of the amino acid sequence represented by Ser-Ala-Leu-Leu-Arg-Ser-Ile-Pro-Ala (SEQ ID NO: 1); (b) an oligopeptide consisting of an amino acid sequence having a deletion, substitution, insertion, or addition of one or several amino acids in Ser-Ala-Leu-Leu-Arg-Ser-Ile-Pro-Ala (SEQ ID NO: 1), and having an activity that inhibits neuronal cell death caused by a mutant superoxide dismutase-1 gene; and (c) a modified oligopeptide from the oligopeptide (a) or (b).