An amorphous object of (3E)-1-[l(s)-1-(4-fluorophenyl)ethyl]-3-[3-methoxy-4-(4methyl-1H-imidazol-1-yl)benzylidene]piperidine-2-one, which is represented by the formula (1) and has Aβ-production inhibitory activity. The amorphous object has such satisfactory properties that it has excellent solubility, is stable, does not readily become a crystalline form, and has low hygroscopicity. It is hence suitable for use in pharmaceutical preparations.