The purpose of the present invention is to provide a novel medicinal agent and a novel method both of which can be used for treating ALS (amyotrophic lateral sclerosis), preventing the progression of the phase of ALS, treating a symptom associated with ALS, or preventing the progression of the phase of the symptom, and both of which can be particularly applied to patients who can be treated by the medicinal agent or the method highly effectively among ALS patients who are needed to be treated. According to the present invention, a medicinal agent which can be used for treating amyotrophic lateral sclerosis, preventing the progression of the phase of amyotrophic lateral sclerosis, treating a symptom associated with amyotrophic lateral sclerosis, or preventing the progression of the phase of the symptom is provided. The medicinal agent comprises 3-methyl-1-phenyl-2-pyrazolin-5-one or a physiologically acceptable salt thereof as an active ingredient, is administered in such a manner that a 14-day dosing period and a 14-day washout period are repeated alternately or in such a manner that a 14-days initial dosing period and a subsequent 14-day initial washout period are set and subsequently a dosing period in which the dosing is carried out for any 10 days of 14 days and a 14-day washout period are repeated alternately, and is administered to a patient who meets specific criteria.
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