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Five parts of a styrene/acrylic acid copolymer (acid value: 250; molecular weight: 5,000) as a dispersing agent were dissolved in 80 parts of dimethyl sulfoxide, and 10 parts of a quinacridone pigment (C.I. Pigment Red 122) were suspended in the resultant solution in a flask at 25° C. under an air atmosphere. A 30% methanol solution of potassium hydroxide was then added dropwise little by little to dissolve the quinacridone pigment. After the pigment solution was stirred for 3 hours, octanoyl peroxide as a polymerization initiator was added to and dissolved in the pigment solution in a proportion of 0.1 parts per 10 parts of the pigment, and the resultant solution was quickly poured into ion-exchanged water (400 parts of ion-exchanged water per 10 parts of the pigment; 0° C.) stirred by an impellor type agitating blade (800 rpm), purged with nitrogen and cooled and temperature-maintained by means of 2 system dispensers (manufactured by Musashi Engineering K.K.; needle internal diameter: 0.57 mm; delivery pressure: 4.0 kgf/cm2) to obtain an aqueous dispersion of quinacridone-pigment-containing particles having an average particle diameter of 35.5 nm. This aqueous dispersion was transferred to a reaction vessel equipped with a mantle heater, cooling tower, stirrer and thermometer. While purging with nitrogen and stirring at 300 rpm, styrene and acrylic acid as ethylenically unsaturated compounds were then added dropwise over 3 hours in proportions of 5.5 parts and 0.5 parts, respectively, per 10 parts of the pigment. The system was then heated to an internal temperature of 80° C. to continue the reaction for 8 hours while retaining the temperature of 80° C. After the reaction was conducted for additional 1 hour at a temperature raised to 90° C., the reaction mixture was cooled back to room temperature.

1. stirringagitating blade (800 rpm)
2. custompurged with nitrogen
3. temperaturecooled
4. temperaturetemperature-maintained by means of 2 system dispensers (manufactured by Musashi Engineering K.K.; needle internal diameter: 0.57 mm; delivery pressure: 4.0 kgf/cm2)