[Object] An object of the present invention is to provide an oral solid preparation that can be produced in a simpler manner than conventional methods, that exhibits high bioavailability and high dissolubility even in persons having low stomach acid, and that can also ensure dissolubility after being allowed to stand for a certain period of time. Another object is to provide a simple method for producing the oral solid preparation.[Means for Achieving the Object] The present invention relates to an oral solid preparation comprising, as an active ingredient, a finely milled crystal obtained by milling an aripiprazole hydrate crystal, and a pharmaceutically acceptable carrier, the finely milled crystal having a mean particle size of 15 μm or less; and a method for producing an oral solid preparation comprising the steps of (1) milling an aripiprazole hydrate crystal into a finely milled crystal having a mean particle size of 15 μm or less, and (2) mixing the obtained finely milled crystal with a pharmaceutically acceptable carrier.