A resin composition having high compositional uniformity can be advantageously prepared in industry by depositing a mixture comprising polyolefin particles and fibrous basic magnesium sulfate aggregated particles which satisfies the condition that a value resulting from calculation of formula of DA2×ρA/DB2×ρB is in the range of 0.2 to 5 in a hopper of a melt kneader through a supply port of the hopper and introducing the deposited mixture into the melt kneader so as to melt and knead the mixture therein, in which DA is a mean particle size (unit: cm) of the polyolefin particles, ρA is a density (unit: g/cm3) of the polyolefin, DB is a mean particle size (unit: cm) of the fibrous basic magnesium sulfate aggregated particles, and ρB is an apparent density (unit: g/cm3) of the aggregated particles.