Crystalline particles of N- (17-cyclopropylmethyl-4,5α-epoxy-3,14-dihydroximorfinan-6ß-yl) -phthalimide or a pharmaceutically acceptable salt thereof, the crystalline particles being finely ground N- (17-cyclopropylmethyl-4) , 5α-epoxy-3,14-5 dihydroximorfinan-6ß-yl) -phthalimide crystalline or one of its pharmaceutically acceptable salts with a fluid energy mill or an impact mill, the crystalline particles having a degree of crystallinity of not less than 85%； and the crystalline particles of any of the following type (a) or the following type (b) being: (a) crystalline particles having a particle diameter distribution at which a particle diameter (D50) at the point where The cumulative frequency of the cumulatively calculated volumetric distribution of particles having a smaller diameter reaches 50% is within a range of 1 to 30 μm, and a particle diameter (D90) at the point where the frequency Cumulative cumulatively calculated volumetric distribution of particles that have a smaller diameter reaches 90% is not greater than 90 μm； (b) crystalline particles having a particle diameter distribution in which the cumulative frequency of the volumetric distribution occupied by particles having a particle diameter based on the volume not exceeding 15 μm is not less than 40%.Partículas cristalinas de N-(17-ciclopropilmetil-4,5α-epoxi-3,14-dihidroximorfinan-6ß-il)-ftalimida o una de sus sales farmacéuticamente aceptables, obteniéndose las partículas cristalinas moliendo finamente N-(17-ciclopropilmetil-4,5α-epoxi-3,14-5 dihidroximorfinan- 6ß-il)-ftalimida cristalina o una de sus sales farmacéuticamente aceptables con un molino de energía fluida o un molino de impacto, teniendo las partículas cristalinas un grado de cristalinidad de no menos del 85 %； y siendo las partículas cristalinas de cualquiera del siguiente tipo (a) o el siguiente tipo (b): (a) partículas cristalinas que tienen una distribución del diámetro de partícula en la que un diámetro de partícula (D50) en el
