A quinolinone derivative pharmaceutical composition comprising a quinolinone derivative represented by the structural formula (I): ##STR00001## wherein the quinolinone derivative is in the form of particles, which have an average particle diameter of 0.5 to 10 .mu.m and such a particle size distribution that particles having a particle diameter of 15 .mu.m or less account for 90% or more of the totality of the particles, and also have a fusion enthalpy of 30 J/g or more, and the surface of the particles are coated with a water-soluble composition containing a water-soluble polymer, can quickly dissolve an active ingredient in the digestive tract and is also superior in long-term storage stability.