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 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.
