The problem addressed by the present invention is to provide a pharmaceutical solid preparation which comprises stabilized [(1R,5S,6S)-6-(aminomethyl)-3-ethylbicyclo[3.2.0]hept-3-en-6-yl]acetic acid monobenzene sulfonate, and to provide a method of preparing said stabilized pharmaceutical solid preparation. The solution is a pharmaceutical solid preparation that contains [(1R,5S,6S)-6-(aminomethyl)-3-ethylbicyclo[3.2.0]hept-3-en-6-yl]acetic acid monobenzene sulfonate, which is a compound having formula (I), and (i) one or more excipients, (ii) one or more disintegrants, and (iii) a specific antioxidant agent.
