An object of the present invention is to provide [(1R,5S,6S)-6-(aminomethyl)-3-ethylbicyclo[3.2.0]hept-3-en-6-yl]acetic acid monobenzenesulfonate as a stabilized pharmaceutical solid preparation, and also to provide a method for preparing the stabilized pharmaceutical solid preparation. The object can be attained by a pharmaceutical solid preparation comprising [(1R,5S,6S)-6-(aminomethyl)-3-ethylbicyclo[3.2.0]hept-3-en-6-yl]acetic acid monobenzenesulfonate which is a compound represented by the following formula (I) in combination with (i) one or two or more excipients, (ii) one or two or more disintegrants, and (iii) a specific antioxidant.