To obtain a magnetic fluid having a prolonged gelling time and improved dispersion stability.

Fine particles of a ferrite having absorbed a water-soluble surfactant are dispersed into a low-vapor pressure base oil solution of an N- polyalkylenepolyamine-substituted alkenyl-succinic acid imide having ≥10wt.% concentration, heat-treated at ≥ about 90°C or irradiated with an ultrasonic wave for ≥ about 100 hours, ≥ about 60wt.% of a water-soluble surfactant previously adsorbed on the fine particles of the ferrite is replaced with the N-polyalkylenepolyamine-substituted alkenyl-succinic acid imide. The water-soluble surfactant is removed together with the low-pressure base oil and the fine particles of the ferrite having adsorbed the N-polyalkylenepolyamine-substituted alkenyl-succinic acid imide are dispersed into the low-pressure base oil to produce a magnetic fluid.