The present invention relates to a magnetic particle-containing water dispersion wherein the magnetic particles have a primary particle diameter of 5 to 15 nm and an average secondary particle diameter of 10 to 60 nm, and the water dispersion has a zeta potential of not more than −20 mV when a pH value of the water dispersion lies within the range of 6 to 8, and further a surface of the respective magnetic particles is coated with a carboxyl group-containing polymer. The magnetic particle-containing water dispersion is produced by heating an aqueous solution in which the carboxyl group-containing polymer is dissolved, to a temperature of 90 to 100° C. in a nitrogen atmosphere; adding a solution of a ferrous (II) salt and a ferric (III) salt and an alkali solution to the aqueous solution to react with each other at the same temperature; adding ethanol to the solution to obtain a precipitate; and removing a supernatant liquid from the solution, and then dispersing the precipitate in water and subjecting the resulting dispersion to dialysis. The magnetic particle-containing water dispersion is useful as a magnetic particle-containing water dispersion capable of producing magnetic particle-containing drugs for diagnosis and therapies which can exhibit a uniform functionality, with a good reproducibility.