Crystalline particles of N-(17-cyclopropylmethyl-4,5α-epoxy-3,14-dihydroxy-morphinan-6β-yl)-phthalimideor a pharmaceutically acceptable salt thereof. Theses crystalline particleshave a particle diameter distribution, wherein the particle diameter (D50)at the point attaining a cumulative frequency of 50% in volume distribution countedfrom the smallest particle diameter ranges from 1 to 30 μm while the particlediameter (D90) at the point attaining a cumulative frequency of 90% in volumedistribution counted from the smallest particle diameter is not more than 90μm, and a degree of crystallization of 80% or more. These crystalline particlesmake it possible to improve the biological availability of the active ingredientwhile ensuring the storage stability of the above-described compound.
