Crystalline particles of N-(17-cyclopropylmethyl-4,5α-epoxy-3,14-dihydroxy-morphinan-6β-yl)-phthalimide or a pharmaceutically acceptable salt thereof are disclosed. The crystalline particles have a particle diameter distribution in which a particle diameter (D50) at the point where cumulative frequency of volume distribution cumulatively calculated from 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 cumulative frequency of volume distribution cumulatively calculated from particles having a smaller diameter reaches 90% is not more than 90 μm, which crystalline particles have a degree of crystallinity of not less than 80%. By the crystalline particles, bioavailability of the active substance may be increased while ensuring storage stability of the compound.